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.
Instead of clobbering the actions with both version and help, use
options instead. They are also going to be used for subcommands (e.g.
omf list --help) once they support options.
