Add some way to configure XKit outside of tumblr pages

[coderpatsy]

Inspired by a recent ask- perhaps XKit could have a control panel in a standalone page or popup, like is already done with the Editor. That way you can still configure it if tumblr goes down or tumblr updates break the control panel button (again).

It might be accomplished through [browser's extensions page] -> Options and/or adding a browser button.

[Wolvan]

I actually was looking into that myself a few weeks ago, I definitely think this is a good idea.