Add handleKeyEvent to Mousetrap object to let follow events

For example when using Mousetrap with Ace Editor, ace capture all the events in the editor and shortcuts from mousetrap are not triggered, using this change and this ace code:

```
var $input = editor.textInput.getElement();
var keyboardShortcutsFollow = function(e) {
    e.mousetrap = true;
    Mousetrap.handleKeyEvent(e);
};
$input.addEventListener('keypress', keyboardShortcutsFollow, false);
$input.addEventListener('keydown', keyboardShortcutsFollow, false);
$input.addEventListener('keyup', keyboardShortcutsFollow, false);
```

It works.