Include support for the splitKey option

[Leroy Witteveen]

I want to be able to bind to the shortcut CTRL and +.
To be able to do this, I need support for the splitKey option (jaywcjlove/hotkeys-js#105).

I think it'd be ideal if it could be used like this:

<Hotkeys keyName="ctrl_+" splitKey="_">...</Hotkeys>

I will try and make a pull request with this feature. :)

[小弟调调]

@leroydev 👍