Support custom parser options in programmatic API

[danez]

Add logicalAssignment to the default plugins.

BREAKING CHANGE: Removed cli arguments --legacy-decorators and --decorators-before-export. Instead use a babel configuration file with the correct options for the transforms.
BREAKING CHANGE: Removed api options legacyDecorators and decoratorsBeforeExport. Instead use a babel configuration file or supply parserOptions and enable the plugins with the correct options.

Do you think this is okay and works with your usecases? I wanted to add this before doing 4.0.

So basically the options are now:

1. No config file and no parserOptions => defaultPlugins
2. No config file and parserOptions with plugins => parserOptions only
3. No config file and parserOptions without plugins => parserOptions + defaultPlugins
4. Config File and no parserOptions => config file only
5. Config File and parserOptions with plugins => babel merges config file with parserOptions
6. Config File and parserOptions without plugins => babel merges config file with parserOptions and defaultPlugins

In all 6 cases estree plugin is now added and tokens set to true, as both are required.

[jquense]

Looks great!