Added `withKeyCode` option to action helper

For actions that are fired on `keyUp`, `keyDown` or `keyPress` an option called `withKeyCode` is now available to use. It ensures that the action is called only when the given `withKeyCode` value matches the keyCode on the event.

For example:

    <input type='text' {{action 'show' on='keyUp' withKeyCode='13'}}>

This action will only fire if the event is a `keyup` and the `keyCode` is 13.

This commit was done with help from: @tonycoco
I think the `test` needs to be wrapped in a `if (Ember.FEATURES.isEnabled("ember-actions-with-key-code")) {`, as otherwise this feature is always enabled/tested and not just when this feature is enabled via feature flags …

cc @rjackson