Add linting config for node based CSS/SCSS linter

[el-mapache]

We currently have a dependency on ruby to handle css linting in most of our front end apps. Ideally we should move to a node based solution. @shawnbot mentioned a tool called 'stylelint' that seems like a good fit. Most of the keys in our scss_lint.yml file map directly to rules in the stylelint library, hooray!

Rules that will require a custom plugin:

• ElsePlacement
• ImportPath
• NameFormat
• PropertyCount
• UnnecessaryParentReference ( don't have the scss foo to write this one without help )
• UrlFormat

We've broken out the stylelint configs into a separate module (here: https://github.com/18F/18F-stylelint) which will eventually be available via npm.

Acceptance Critera:

• [ ] Add a helpful description of the new linter
• [ ] Link the new css linting config file on the front-end guide

[shawnbot]

Should we deprecate the scss-lint suggestions and config?

[msecret]

@shawnbot that's what I want to do; we might want to fix the ignore config issue on stylelint-rules soon though.