Make screen view url plural

Currently the API and display routes are a bit unintuitive, since one is plural and the other singular.

- To get info on a screen you make a request to `/api/screens`
- But to to view a screen you go to `/screen/NAME`

With this patch I propose to change the viewing route to `/screens/NAME` to present a more consistent experience.

(It also maintains backward compatibility with the old singular route, but changes the README since /screens/ should be preferred.)

Let me know if you have any questions or problems with this!