Add mapping of CodeMirror events to actions

[omghax]

This adds an events property which allows you to declaratively set up CodeMirror event handlers from your templates, like so:

{{ivy-codemirror events=(hash change=(action "onChange"))}}

The onChange action would then be invoked whenever a CodeMirror change event occurs, passing along any arguments from CodeMirror. For now I'm assuming that events is unbound, so changing it after the initial render would not cause the event handlers to be updated.

[omghax]

This would be a breaking change, and would necessitate a 3.0 release.