Add highlighting-follows-the-cursor-on-variables

As seen at:

  http://blogs.perl.org/users/ovid/2014/05/automatic-variable-highlighting-in-vim.html

And living at:

  https://gist.github.com/Ovid/386157c880221ac72fbc

  Commit id: cc601f142e187f5992aaa20908507f0d8466a403

Thanks, @Ovid!