[amp-story-player] Feature flags

As we develop new features for the player, it would be good to be able to use either AMP experiments or some other construct to hide unfinished or experomental features behind a flag.

This will give us the flexibility to e.g. make API changes before "launching" a feature, without making backwards-incompatible changes.

Note: It's possible that the AMP experiment framework might work out of the box; I haven't tested it.