Merge: vim: allow spell check on string literals and comments.

Just `:set spell` to see errors, and `z=` on highlighted words to see
suggestions.

Pull-Request: #643
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Frédéric Vachon <fredvac@gmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>