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 :-/.
TODO:
* `git.enable = false` would break the standalone configuration
* using a wrapper which sets `--config` might be better than includes
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.
