Recently viewed (client side history)

See also #51 discussion

This probably needs a little bit more work and it doesn't work with DJ mode.

It would be nice to have dates next to the names, but the built-in Date object doesn't have a format method built in. 

### Open questions
* Should we also preserve playlist order?
* Do we need a clear history button? If so, where to put it?

Please review. :smile: