Add initial command to end of completions

This means that if the user cycles through all completions then they
will see the initial string they entered.

aerc.go

@@ -78,6 +78,7 @@ func getCompletions(aerc *widgets.Aerc, cmd string) []string {
 	for _, set := range getCommands((*aerc).SelectedTab()) {
 		completions = append(completions, set.GetCompletions(aerc, cmd)...)
 	}
+	completions = append(completions, cmd)
 	return completions
 }