Track most commonly used commands for more informative help

[mlncn]

(Sorry, can't tag things Feature request myself)

Limnoria attempts to be self-documenting. For people interacting with an already set-up bot, it would particular helpful for them to see a list of that bot's most-used commands.

For someone coming in who just wants to interact with the bot, the commands nested in plugins are far from self-documenting. If the bot kept track of most-used commands (does it already?), and in its "help" listed those with definitions, it would be pretty close to self-documenting for new people entering a channel.

For extra fanciness this list of popular commands could be filtered to the asking user's permission/role, or based on the commands used by other users with the same permission/role. But one overall list per bot and/or per channel would be really helpful in auto-generating a basic intro that otherwise would have to be manually built by each community using the bot.