- Implement MultiEditing for :NRM
  (see https://github.com/felixr/vim-multiedit/blob/master/plugin/multiedit.vim
   and http://blog.felixriedel.com/2012/06/multi-editing-in-vim/)

- When disabling Narrowing using :NRN and one quits the buffer,
  should the highlighting be removed? (it doesn't yet)

- I really need some tests, like:
  * Visual Narrowing
  * Narrowing 1 Region
  * Narrowing several independent regions with setting the g:nrrw_rgn_protect
    variable
  * Multi line narrowing