Feature: API to set the current element programatically

[argyleink]

If the keyboard / roving-ux didnt change the focus, how can this library get the update so it's state can match the new state. could just be exporting active() for users to call themselves? Would also be nice to not have focus called too, so an extra parameter would be nice.

todo:

• [ ] decide on an api function to export
• [ ] add the extra parameter whether or not focus should be called, can default to todays behavior
• [ ] add a test or 2
• [ ] update the docs