OPTION: Block unpopular accounts (< 15 followers)

Introduce `block_low_followers` option; implementation similar to `block_new_accounts`
Non-customizable implementation of Issue #39
Can easily rebase on top of #57 if accepted
I think "unpopular" conveys the wrong tone here. The goal (in my mind at least) is to block probable sockpuppets / new accounts, when the 7 day filter is no longer doing the job. I would just leave this as "Block accounts with < 15 followers that mention you."