Ideally, I'd like for `delta` to just read a configuration file at
`$XDG_CONFIG_HOME/delta/config` by default, but upstream seems somewhat
reticent to the idea :-/.
So instead, let's keep relying on `git` being enabled, but rather than
inlining the configuration, let's store it where I think it should
belong and include it into `gitconfig`.
I want to be able to re-use it between different source control systems
(e.g: `jj`).
As a first step, extract it to a proper module so that I can have it
live in a single space.
