Refactor _suiteForType helper.

[mattcollier]

I think we're playing a little fast and loose with this API (maybe you pass a doc, maybe you pass some options, if the thing you pass isn't defined its treated the same as no type being passed ... and so on). I could see someone getting tripped up on this in the future. So we might want it to be a little cleaner, but won't hold up the PR for it.

Originally posted by @dlongley in #60 (comment)