Add a version class to the player

[mister-ben]

Description

Add a version class to the player to simplify CSS for plugins supporting multiple versions. For example to add an icon to a button in v5 you'd target .my-button:before, in v6 .my-button .vjs-icon-placeholder:before.

Specific Changes proposed

Adds a class e.g. vjs-v6 to the player element.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chome, Firefox, IE)
  • Unit Tests updated or fixed
• Reviewed by Two Core Contributors

[gkatsev]

LGTM.
Did we want to make one against 5.x as well? It won't be as useful but maybe still.