Refactor how commands and help interact to build usage.

[rgroves]

This is an good start, but it'd be a bit cleaner if help had a method to allow commands to register with help as they are loaded by the bot, instead of help having to have knowledge of the command files and having to require them to build the help.

Originally posted by @rgroves in https://github.com/tomassirio/ListBot/pull/162#discussion_r546108699