Add new onInput event

The `input` event is supported in IE9+ and all other browsers according to https://developer.mozilla.org/en-US/docs/Web/API/window.oninput. onInput has the advantage that it responds to repeated key events before onKeyUp and is also called when modifying the input without the keyboard (such as pasting with the mouse).

I also modified the two examples that use onKeyUp (markdown, ballmer-peak) to use onInput but am happy to revert that commit if for some reason you prefer to keep using onKeyUp.