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

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I am not sure where this should be. Here are some options:
- `editor.less` (the current implementation in the PR).
- A new `underlays.less` because `.wrap-guide` is in `div.underlayer` and we already have `overlays.less`.
- A new `wrap-guide.less` because it is for a specific package.