Expose query's selected fields to resolvers via context

[keithmattix]

Since #169 seems to have stagnated a bit, I decided to throw my hat in the ring. I'm not sure how everyone feels about exposing this information via context vs. a third parameter, but hopefully this can serve as a starting point for more discussion on the issue. Fixes #17

[willglynn]

I would rather have field selections exposed as an argument (#169) than via Context (this PR). If my resolver needs field selections, it's because there's logic in my resolver which depends on those field selections, and I'd prefer to state that explicitly in my resolver's function signature. My resolvers' use of Context typically doesn't affect control flow, while field selection always does.

Having said that, I would rather have field selections exposed via Context than not exposed at all. This kind of functionality is important to me, and regardless of how it's exposed, merging either PR would allow me to stop using a fork.

[dusansimic]

This feature is really useful to me and it'd be very nice if it gets pulled. I hope this will be done soon.

[keithmattix]

Closing in favor of #428