Feature: Optionally allow hotkeys in form elements

By default, Mousetrap prevents hotkey callbacks from firing when their event originates from an `input`, `select`, or `textarea` element. This PR introduces a new parameter for hotkeys.add() called `allowIn` that allows the user to specify an array of elements ('INPUT', 'SELECT', and/or 'TEXTAREA') for which the hotkey will be permitted in. Note that this hotkey will be triggered when the event originates from any other element *or* the elements listed in `allowIn.` The new parameter is simply a way to enable hotkeys for  `input`, `select`, and/or `textarea` elements.