Add triggers support to behaviors

I started looking into #1642 because I recently needed the functionality.  This was my initial attempt to implement triggers for behaviors without duplicating too much code.  At first glance, this should be BC.  It still needs more specs and the first `calledWith` needs fixed.  I still need to match the expected `args` parameter, but I'm getting max call size.

Need lots of eyes and feedback on this one because behaviors are still relatively new to me too.  It's semi-modeled after the implementation for `behaviorEvents`.

Note this is also introduces `Marionette.buildViewTrigger` and `Marionette.proxyBuildViewTrigger` to avoid code duplication from `View#configureTriggers`, so I'm not sure how we feel about that.
What if `triggerDef` is a string like `'clicked:foo'`?