Add a lint for non-idiomatic identifier names

[nex3]

Sass allows either underscores or hyphens to be used interchangeably as identifier separators, but hyphens are preferred as they match CSS's own naming conventions. We should have a lint for any Sass identifier (variable, mixin, function, or placeholder name) that complains about underscores and suggests using hyphens instead.

[nex3]

It's probably also a good idea to complain about camel-case names.