Synchronize volume across player types

This feature reads the volume level of the current player before transitioning to a new video, to prevent jarring changes in volume when the player changes e.g. from YouTube to Soundcloud.  Furthermore, the volume is saved in `localStorage` and persists.

Known, unfixable issue with Soundcloud: Soundcloud's volume cannot be changed until playback begins.  Because of this, it is possible that a short (< 1 second) clip might play at maximum volume before the volume is set.  How they managed to screw this up is beyond me.