[feature suggestion] Basic Regex Filtering / Spam Filter

With the increasing number of the random digit posts, I think it's evident PyBitmessage could benefit from some sort of filter to help ignore these spammy messages on the client side that's user configured.

The easiest implementation I can think of is a text file in the config directory that lists regular expressions on each line to apply to incoming messages to ignore them. This would be up to the user to configure manually.

While it would be more work, it would be nice to see this feature in the GUI Blacklist tab, when you click the "Add new entry" button. Or a new "Spam" folder in the Messages -> Identities pane that would redirect these messages similar to most email clients.

I think most people could come up with even more user friendly functionality, but hopefully the first suggestion (with a config file) can demonstrate this is feasible even at the most rudimentary level.