Highlight button when focused; add changeResolutionSrc(); add videojs-contrib-ads compatibility

* Highlight menu button when focused (e.g. by keyboard).
* Add `player.changeResolutionSrc(sources)` to change the source after the plugin's already been initialized.
* Add videojs-contrib-ads compatibility to `player.changeResolution()`. Assumes videojs/videojs-contrib-ads#28 is accepted.