Use triggerMethod on child view if available

This PR fixes #1488.  It abstracts out the logic a collection view uses for calling `triggerMethod` on a child view.  If the child view implements `triggerMethod`, then it calls that.  Otherwise, it calls its own implementation on the child view.

I cleaned up one of the specs that was kind of related, but we probably want specs that actually verify that the correct `triggerMethod` is called.

Please look over this to make sure this satisfies the requirements of #1488 and isn't too crazy :sleeping: 
@jasonLaster I completely forgot about #1558 (and I had :+1:'d it too).  I'm definitely for using it over a custom method.

@marionettejs/marionette-core Should we merge that first into 2.1 and then rebase this PR? Use this PR and create a separate issue to incorporate #1558?  What do you guys prefer for this type of situation?