Implement over-screen tab search UI

[jchang504]

We talked about this a bit during the hackathon. Would be slick to have a over-screen modal UI for the tab search, rather than just opening it in a new tab as we do currently. This requires the background script sending tab info to the content script, but we've got lots of back-and-forth communication going on already, doesn't seem too hard to add.

[jchang504]

This is now necessary to resolve the first bug mentioned in 2d78d54. It would go well with an implementation of the over-screen hold key/hotkey indicator that we're thinking about as a pseudo-solution to #15, and it would eliminate #16.