Generate Video Sprites

Would be cool to (optionally) generate a sprite for movies / episodes to be displayed as you hover along the progress bar. This is useful for finding a specific spot in a movie you like etc etc. Also a stopgap until there's supports for chapters.

This should by default be false, as it would bloat a library with images even if they are very small. But i'm envisioning turning it on per library, per series etc.

See https://github.com/phloxic/videojs-sprite-thumbnails for the plugin (updated recently!) and examples.

Related to #220 as starting either would make the other easier in terms of taking a screenshot at a specific time.