mutt: Add "O" macro to save original message

Because I can't remember the trick mentioned in 78e9ef2.

roles/dotfiles/files/.mutt/config/macros.mutt

@@ -57,6 +57,7 @@ macro pager *u "<exit><untag-pattern>~T<enter><tag-pattern>~U<enter>" "select un
 macro index,pager \\u "<enter-command>set pipe_decode = yes<enter><pipe-message>urlview<enter><enter-command>set pipe_decode = no<enter>" "view URLs"
 
 macro index,pager | "<pipe-message>" "pipe message to..."
+macro index,pager O "<pipe-message>cat > ~/Desktop/original-msg.txt" "save [O]riginal message"
 
 # [m]ove a mail to another folder (in Gmail, "m" would be "[m]ute").
 macro pager m "<save-message>?<toggle-mailboxes>" "move to..."