New 'input' event type for user input

Currently 'content-changed' is called when setHTML() is called. Typically 'change' events aren't fired after setters are called. For example, the HTML input field's input/change events aren't fired when the 'value' attribute is changed manually using JS.

It would be nice if Scribe had its own 'input' and 'change' events that only fired when the user did something, e.g. typed, undo, paste, etc.