Add before and after suite

Addresses issue https://github.com/modocache/Quick/issues/2. 

Adds optional closures that can be run before and after the suite.

Comments are inline. I'd love to get feedback!
You have to return `null`? That's actually news to me, haha!

Is the rule that the last statement in a closure is returned as the closure's return value, but only when the closure is a single line long?