Propagate preventDefault() to original event

This is related to #4. If I have a custom event handler for `tap`, `doubletap`, `flick`, `drag`, etc. that calls `event.preventDefault()`, then Finger should call `event.preventDefault()` on the **original** event that triggered `stopHandler()`.

This allows you to prevent the default behavior (native clicks, text selection, etc.) but *only* in certain cases (i.e., where a Finger event was handled and that handler prevented the default action).

Since this only affects cases where the `tap` (or whatever) handler calls `event.preventDefault()`, I don't think this even needs an option in Finger (obviously, if you want the default behavior, you should avoid calling `event.preventDefault()`).