Add postpone command

This command uses the Postpone folder from the account config to save
messages to. Messages are saved as though they were sent so have a valid
'to' recipient address and should be able to be read back in for later
editing.

commands/msg/forward.go

@@ -70,7 +70,6 @@ func (forward) Execute(aerc *widgets.Aerc, args []string) error {
 		"To":      to,
 		"Subject": subject,
 	}
-
 	original := models.OriginalMail{}
 
 	addTab := func() (*widgets.Composer, error) {