fix(emacs/lean-mmm-lua): add 'insert' for mmm-insert-region (C-c % l)

Related issue: #74, https://github.com/flycheck/flycheck/issues/349, https://github.com/purcell/mmm-mode/issues/32
fix(emacs/lean-mmm-lua): add 'insert' for mmm-insert-region (C-c % l)