Add Per-User configuration settings for setting specific per-user configuration options

[onerandomusername]

It would be nice to be able to configure a GitHub organisation that I, as a user can use across Discord. Or maybe I don't want the bot to reply to me when I send a discourse URL. Or maybe I want my docs command to only show inventories from specific packages!

Users should be able to set settings on a per-guild basis, with guild command able to override if the guild chooses that option.

Deciding how guild configuration intersects with user configuration still needs to be thought about.