In order to pro-actively diagnose an issue on someone's computer, a
`omf doctor' command is being added. This command will initially check
errors that could cause a theme to not be loaded, but in the future more
checks are going to be added to prevent issues from being created for
problems that are fixable by the Oh My Fish user alone.

Per conversation with @bpinto in Gitter.

There's no need for two separate directories. You don't have a `.git` and `.git-custom` folder, you just put your config in `.git` :)

The most straightforward interpretation of XDG basedir spec is that user configuration for omf would go in `~/.config/omf`, so let's put it there. The only question is whether omf-generated config (i.e. the `theme` file) should go there as well. By analogy with git, programmatically generated config should probably be merged in with user config. This also makes it so when a user clones their dotfiles to a new machine, both kinds of settings come with it.