Plugable annotation system added

I add the possibility to extend ngmin with annotation packages which are prefixed with `astral-angular-annotate-`. One extension can be found here https://github.com/werk85/astral-angular-annotate-ng-route. This adds `resolve` support to the ngRoute annotations.

The target of this pull request is to make ngmin more open and allow external angular libraries to provide their own annotations. This would resolve many issues here in the issue tracker. Also would it be possible for the angular team to split up their annotations in separate packages.

This is not finished yet, but it would be nice to get some feedback.

TODO:

- [ ] Add documentation to the README

Thanks to @sindresorhus who inspired me to this implementation.