Add support for custom subreddit sources

[lydia]

There are various pretty subreddits that would make lovely screensavers. Though a user could certainly fork the repo and add whatever they desire (as I have with limited success as a non-JS dev here), that is asking rather a lot of people.

[opus1269]

Lydia,

I want to add an interface that will let the user specify subreddits by name. So much to do, so little time :)