neomutt: add support for signature command (#2899)

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>

tests/modules/programs/neomutt/hm-example.com-gpg-expected.conf

@@ -30,4 +30,5 @@ set trash='+Trash'
 
 # Extra configuration
 
+
 unset signature