Add support for shared configuration in `~/.config/cogs/`

[jamesaoverton]

I've been thinking about this for a while and haven't made much progress, but I think we want some sort of configuration for the user in a standard place, such as ~/.config/cogs/. This should include an INI-style or YAML file or something for global settings, and one or more credential files. The main pain point right now is making it easier to manage configuration files for multiple projects. There's a lot of possible solutions but I don't know which is the right design. As always, we should look to how git does things for guidance.

[jamesaoverton]

GitHub is acting up... Closing in favour of #97.