Accept IETF language tags, plus accessible play/pause button -- "iet-ou/cr1262/a11y"

Related bugs: #1262, #1263.

Hi John,

Please pull these three commits. Note, I haven't rebuilt `mediaelement-and-player.js` etc. - I wasn't sure what the etiquette was.

I hope this is useful. The first of several! Thanks,

Nick