Rework plugins

[olov]

The plugin API is super-experimental haven't been kept up to date. The API as it once was created focused a lot on extensibility it terms of matching rather than providing usable output for other tools (the latter is probably more useful).

The API as it looks right now should most likely be deprecated in favor of a new, improved (and documented) API. I welcome contributions in this area!

[diwu1989]

I just wrote a plugin today and I had to look into the source code of ngAnnotate and ngInject to learn how it works, but it didn't take too much effort to write a plugin for what I needed. I think if you just had a few examples of plugins for people to look at, then it'd be a lot easier to use. The current API seems okay, having examples would have been enough for me.

[vatula]

I found it almost okay. The usage of custom attributes on nodes like $chained etc is obscure though and I still don't understand what needs to be returned by the plugin. A function that needs guards?

[olov]

#245