Add auto-advance behavior

[azaroth42]

Duplicating from https://github.com/IIIF/iiif-av/blob/master/source/api/av/index.md for documentation purposes.

• Add a new viewingHint value of "auto-advance", that can be associated with Collection, Manifest, Sequence and Canvas. When the client reaches the end of a Canvas with a duration dimension that has (or is within a resource that has) this viewingHint, it SHOULD immediately proceed to the next Canvas and render it.
  If there is no subsequent Canvas in the current context, then this viewingHint should be ignored.
  When applied to a Collection, the client should treat the first Canvas of the next Manifest as following the last Canvas of the previous Manifest, respecting any startCanvas specified.

[azaroth42]

Closing, done at some point in the depths of github history. Don't git-blame me.