Adding "fluid" mode

This is a first pass at the ideas discussed in #982.
If 16-9 is the default we should probably just include that padding in the vjs-fluid class. Otherwise vjs-fluid alone will hide the player, right?

For 4:3 I'd be fine with shortening the class name to vjs-4-3. I think `vjs-fluid vjs-4-3` reads well.