Filterable: Document events beforefilter and filter

Fixes gh-224
Fixes gh-225

Note: The documentation for the fitler event generates a note saying the "ui" parameter is empty, despite the fact that it is not of type object, but of type jQuery. See https://github.com/jquery/grunt-jquery-content/issues/48 and https://github.com/jquery/grunt-jquery-content/pull/49.