Allow @imports of *.css files

Currently with this plugin I can't have an import like "foo.css" because this plugin adds *.scss to the end of the file path regardless. Example: `@import foo.css` in a sass file will fail attempting to load `foo.css.scss`. This commit should fix this issue.

The fix checks to see if there is already a file extension appended to the `match_result` argument in `_get_path()`. If there is and it's `.scss` or `.css`, it won't append another extension on the import_path. If no extension exists, or it doesn't match `.css` or `.scss`, then `.scss` will be added to the import_path.