add a feature to improve the experience

First of all — big thanks for awesome extension, i'm very happy with it!

But I found one obvious and at the same time nasty issue, I try explain that on my experience:
How i'm listening music: new music i looking on the soundcloud, but when I want to listen playlist of my favourite tracks — i'm going to vk.com, therefore i'm listening music on two sites during the day. VK is not only huge base of music, it also best social network in Europe at this moment; and i spent a lot of time on it. When I'm sitting on VK, but listening music on soundcloud — **media keys control players on both sites simultaneously** and it's a problem.

I see solution in minimalistic ui element (icon at the browser panel aka browser_action) button with current site icon, which will mean "I want media keys work only with this site". But if there are a problems with inserting icon of current site each time to that button, we could implement  something else. 

What do you think, @berrberr?

By the way I can figure out with codebase and try implement by myself, then send PR.

sorry for my english =D