Guarantee new backgrounds each day

[rconradharris]

If we keep a little bit of state, we can ensure that a new background is provided each day.

Maybe this can be a config, like ensure_fresh=1.

This would work by appending image URLS to a ~/.reddit-background.log file. The file would behave like a circular buffer of size 100, and would prune itself after each run.

The default for ensure_fresh should probably be true.