Allow use of other keys as modifier keys

It would be nice to use other keys (besides just Shift, Ctrl, Alt, Windows) as modifier keys. For instance, when I used to use kmonad, I was able to make the spacebar a modifier key. I would set a timeout, and if I held the spacebar for longer than the timeout time, it acted as a modifier. I would use it to turn my H, J, K, and L keys into arrow keys (like they are in VIM) so that I wouldn't have to take my hands off the home row in order to move my cursor in other applications. So holding down the spacebar and then hitting J would act like a down-arrow key. It was really useful.