Avoid setting global nomodifiable in selector help (#248)
Avoid setting global nomodifiable in selector help (#248)

* Configure nomodifiable with `:setlocal` instead of `:set` or `:let` to
  make sure it's not inherited by other opened windows.
* Simplify options handling in `selector#DoToggleHelp()`.

Selector buffer should be readonly and nomodifiable unless we need to
change its content.
malcolmr/vim-maktaba,google/vim-maktaba