Use the file extension to check file types

This prevents issues with imported code that may use extensions like .css.js

Fixes #31

src/plugin.ts

@@ -21,7 +21,7 @@ export function buildEnd(err?: Error): void {
 }
 
 function useStyleSheetTransform(filename: string) {
-  return filename.match(/\.s?(a|c)ss/);
+  return path.extname(filename).match(/s?(a|c)ss/);
 }
 
 function useTypescriptTransform(filename: string) {