Add JavaScript hook for altering inserted content

I'm currently working on a feature request over in the Caption Filter queue to make it so that module will utilize Insert if available to insert inline captions (and support floating the image to the left or right). In order to support that module though, we need the ability to modify the inserted content at the time the "Insert" button is clicked. Similar to how we already do a fair bit of processing (replacing the __description__ token for example) off of the existing template, other modules need the ability to modify the content in the same way.

This patch adds a new JavaScript hook in the same vein as Flag module, registering a global event that other modules may respond to.

The Caption Filter issue: #1282078: Add support for Insert module templates