Add self-configuration verbs

[mjcheetham]

Feature description

Rather than having platform/distribution-mechanism specific post-install configuration logic, we should just move this to .NET code, and introduce some new CLI verbs that can be easily invoked from the installers etc.

• configure/install:
  • Configure ourselves as the credential helper, ideally in user/global config.
• unconfigure
  • Remove GCM from the configured credential helper. Should we reset to previous or platform default? Just remove our setting?
• uninstall
  • Invoke the uninstall script on Mac and invoke the Inno uninstaller on Windows?