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
The simplest way to DRY this up is probably to move the reload up a level to surround both cases (new account or low followers). That's how I had it originally, in anticipation of adding features like this, but I found that the catchup code (get 50 @-replies at startup) triggered what felt like too many queries. However I suspect that's a bias caused by logspam, and not actually a performance issue. I should really tweak the log level to be lower for prod (need to move log4js to a config file setup, with different configs prod vs dev).