Add new-email trigger

This patch sets up the trigger config section of aerc.conf.

Each trigger has its own function which is called from the place where
it is triggered. Currently only the new-email trigger is implemented.

The triggers make use of format strings. For instance, in the new-email
trigger this allows the user to select the trigger command and also the
information extracted from the command and placed into their command.

To actually execute the trigger commands the keypresses are simulated.

Further triggers can be implemented in the future.

Formatting of the command is moved to a new package.

commands/commands.go

@@ -56,11 +56,7 @@ type CommandSource interface {
 	Commands() *Commands
 }
 
-func (cmds *Commands) ExecuteCommand(aerc *widgets.Aerc, cmd string) error {
-	args, err := shlex.Split(cmd)
-	if err != nil {
-		return err
-	}
+func (cmds *Commands) ExecuteCommand(aerc *widgets.Aerc, args []string) error {
 	if len(args) == 0 {
 		return errors.New("Expected a command.")
 	}