expose bots hash for editing from config block

Basically with the long list of bots it seems better to not be replacing the whole regex, rather just adding individual bots as-needed (a redis-managed solution might ultimately be best, but this is a quick hack). This just lets you do something like:

Split.configure do |config|
  config.bots['my_bot'] = 'Our monitoring bot'
end

I moved robot_regex to a method because otherwise it memoizes the escaped_bots output on initialization.