Localization

Stub support for localization
Yeah, so PlayToggle updates the label in a very ugly way.
https://github.com/videojs/video.js/blob/v4.6.4/src/js/control-bar/play-toggle.js#L37

It should at least be using the controlText_ property there.
https://github.com/videojs/video.js/blob/v4.6.4/src/js/button.js#L46

But better yet would be if we created an `updateLabel` function for the main button class that includes localizing in it, that subclasses could use to change the label. Want to try that?

Any other examples?