Implement "Import Path" lint rule

[srawlins]

Need to implement a lint rule that reports import paths that do not fit the brief, human-readable style. Namely, underscores and file suffixes are unnecessary.

@import "partials/_colors.scss";  // LINT
@import "partials/_colors";       // LINT
@import "partials/colors.scss";   // LINT

@import "partials/colors";        // OK

This is similar to scss-lint's ImportPath.