Add help and manual flags, use pflag

THIS IS A BREAKING CHANGE

Switch flags implementation to use pflag.  This means that long flags
like `-username` must now use 2 dashes: `--username`.

The `-v` flag (verbose) used to accept `-v` or `--v`.  Now it only
accepts `-v.

The `--help` and `-h` flags are new.

The `--man` flag is new (print a man-page like help message).

Several glog flags which used to be exposed (e.g. --logtostderr) are no
longer exposed.  Logs always go to stderr.

Author: thockin

README.md

@@ -94,14 +94,14 @@ docker run -d \
 | GIT_SYNC_ONE_TIME               | `--one-time`               | exit after the first sync                                                                                                                                                                                                                     | false                         |
 | GIT_SYNC_MAX_SYNC_FAILURES      | `--max-sync-failures`      | the number of consecutive failures allowed before aborting (the first sync must succeed, -1 will retry forever after the initial sync)                                                                                                        | 0                             |
 | GIT_SYNC_PERMISSIONS            | `--change-permissions`     | the file permissions to apply to the checked-out files (0 will not change permissions at all)                                                                                                                                                 | 0                             |
-| GIT_SYNC_HOOK_COMMAND           | `--sync-hook-command`      | the command executed with the syncing repository as its working directory after syncing a new hash of the remote repository. it is subject to the sync time out and will extend period between syncs. (doesn't support the command arguments) | ""                            |
+| GIT_SYNC_HOOK_COMMAND           | `--sync-hook-command`      | an optional command to be executed after syncing a new hash of the remote repository                                            |
 | GIT_SYNC_WEBHOOK_URL            | `--webhook-url`            | the URL for a webook notification when syncs complete                                                                                                                                                                                         | ""                            |
 | GIT_SYNC_WEBHOOK_METHOD         | `--webhook-method`         | the HTTP method for the webhook                                                                                                                                                                                                               | "POST"                        |
 | GIT_SYNC_WEBHOOK_SUCCESS_STATUS | `--webhook-success-status` | the HTTP status code indicating a successful webhook (-1 disables success checks to make webhooks fire-and-forget)                                                                                                                            | 200                           |
 | GIT_SYNC_WEBHOOK_TIMEOUT        | `--webhook-timeout`        | the timeout for the webhook                                                                                                                                                                                                                   | 1 (second)                    |
 | GIT_SYNC_WEBHOOK_BACKOFF        | `--webhook-backoff`        | the time to wait before retrying a failed webhook                                                                                                                                                                                             | 3 (seconds)                   |
 | GIT_SYNC_USERNAME               | `--username`               | the username to use for git auth                                                                                                                                                                                                              | ""                            |
-| GIT_SYNC_PASSWORD               | `--password`               | the password or [personal access token](https://docs.github.com/en/free-pro-team@latest/github/authenticating-to-github/creating-a-personal-access-token) to use for git auth. (users should prefer env vars for passwords)                                                                                                                                                                 | ""                            |
+| GIT_SYNC_PASSWORD               | `--password`               | the password or personal access token to use for git auth (prefer env vars for passwords)                                                                                                                                                     | ""                            |
 | GIT_SYNC_SSH                    | `--ssh`                    | use SSH for git operations                                                                                                                                                                                                                    | false                         |
 | GIT_SSH_KEY_FILE                | `--ssh-key-file`           | the SSH key to use                                                                                                                                                                                                                            | "/etc/git-secret/ssh"         |
 | GIT_KNOWN_HOSTS                 | `--ssh-known-hosts`        | enable SSH known_hosts verification                                                                                                                                                                                                           | true                          |