Add secure user-configurable modeline support

Modelines are not enabled for root users in Vim 7 because they aren't
secure by default. This plugin fixes that security issue and allows
modelines to work for root users.

See vim script 1876 at
http://www.vim.org/scripts/script.php?script_id=1876

See github repo at https://github.com/ciaranm/securemodelines