Add support for reusable configuration and credentials in `~/.config/cogs/`

[jamesaoverton]

My main pain point with COGS right now is that it's a bit of a pain to just start using it for a new task. I'd like to make it easier to manage credentials files for multiple projects and just use them, without looking around my projects and copying credentials files around.

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. 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.