Add preference to hide extinct species (etc.)

[Adam Winer]

Original [issue 11](https://code.google.com/p/scythebill-birdlist/issues/detail?id=11) created by adam_winer on 2010-11-05T00:54:18.000Z:

Extinct species make me sad. Especially Mohoidae. Add a user preference to assuage my troubled mind by hiding these.