Colorize `.wrap-guide`

Giving some color to `.editor .wrap-guide` by default is nice.

The [wrap-guide package][wrap-guide] does not set any `background-color` by default and therefore one might think it is not working.

Since wrap-guide is an official Atom package I think it makes sense the default theme to support it.

[wrap-guide]: https://github.com/atom/wrap-guide