New loading icons and class to center big play button

## Icons
- 3 new loading spinners to give people a few options (as well as changed the default).
- Added a share icon

## Optionally centered big play button
- Added a class to allow for centering the big play button

## Docs
- Added more information about the icon font
- Added information on using `.vjs-big-play-centered` to the setup guide.