Update _settings.scss for additional Side Nav variables

Hey guys and gals,

I noticed today that a few of the Side Nav variables created in: https://github.com/zurb/foundation/commit/b33075351904bdb8b9404af6eaad0d843ca3b3d2 weren't in _settings.scss so I've added them in this PR.

Even though is only a small PR I did just spend the last 3 hours trying to get
`bundle && bundle exec foreman start` to work in the dist/docs folder as per the [guidelines for contributing](https://github.com/zurb/foundation/blob/master/CONTRIBUTING.md#contributing). I'm on a Windows box though so I tried rolling back different versions of Ruby and installing the RubyDev kit but I didn't have any luck :cry:

Apologies for not being able to do that so here's a plain old PR :smile: