continued support for importing vanilla CSS files

[FND]

Importing .css files has been deprecated: sass/libsass#2613 (rationale in sass/libsass#2611)

We want to continue to support this (e.g. to bundle non-Sass libraries), so we should either create a custom importer or specify .css as an additional extension (preferably the former to bypass Sass processing).

[xzyfer]

Not using additional extensions does not bypass sass processing. All extensions are parsed as Sass files.

[FND]

Thanks for chiming in here, @xzyfer! Unfortunately, we're not quite sure yet what this actually means in practice: sass/libsass#2611 states that code importing vanilla CSS files "will not be portable" - but isn't CSS a subset of Sass anyway, so what might actually go wrong as a consequence of treating *.css as Sass modules?

We wouldn't want for faucet to depart from or even work against Sass norms, but we're uncertain what the right way is to proceed here.

[mkhl]

For tracking purposes: sass/node-sass#2362

[moonglum]

This is done with Release 1.0.0-rc.9 by switching from node-sass to sass.