build(css): use sass

[vltansky]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

This is a draft, for now, relevant to #2274

[lgtm-com]

This pull request introduces 1 alert when merging 3f1c7e6 into c3189a6 - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[roblarsen]

This approach doesn't make sense to me. It's just extra authoring steps. We author main.css in another repo, so exposing it like it should be editable here is confusing. Also, as I said in the main.css repo there isn't any benefit to using sass with these files. I was literally searching for features I could use to justify it and there were none, so I abandoned the idea.

I would potentially be interested in offering this an an option for end users using the create-html5-boilerplate project.