Neomutt will run the given command (which can be a string or a path)
and take the output from stdout and use it as the signature for your
email.
Co-authored-by: Nicolas Berbiche <nicolas@normie.dev>
The `accounts.email.accounts.<name>.neomutt.extraConfig` option is
included twice in the resulting config file for the account. One time as
part of the `mraSection`, one time as part of `accountStr` (`accountStr`
includes the `mraSection`). This removes that duplication. I opted to
keep the one in `accounStr`, since `extraConfig` doesn't necessarily
have anything to do with the `mraSection`.
