Auto clean old cards && concards default behavior

[alanxoc3]

If some meta data hasn't been passed off for a certain amount of time, it should be cleaned out. So if the next review date for a card was like 100 days ago, it should just be cleaned out. Of course this could be configured.

Edit: Coming back to this after a few months. This should not be a default, instead there should be an option that will run this. --auto-clean <num>. This means I should also change the default for concards to do nothing if no files are supplied (so it shouldn't error out).