Modified game_loop_html.dart to work cross-browser.

I've made a couple of small changes to `_processInputEvents`.

1. Changed `mouseEvent.movement` to `mouseEvent.client`, allowing mouse input to be cross-browser compatible.
2. Added bool variables to determine user need for input events; only checks for event changes if the user wants to track them. This can probably be more elegantly done.
3. Put in an example of enabling only certain events to the test html. This is for your benefit and should be modified to enable all the events you've written code for.