allow additionalData to be async

This is especially useful if you want to use `loaderContext.resolve` to get additional data through the standard webpack mechanisms.

Author: brandonbloom

src/index.js

@@ -33,7 +33,7 @@ async function lessLoader(source) {
   if (typeof options.additionalData !== 'undefined') {
     data =
       typeof options.additionalData === 'function'
-        ? `${options.additionalData(data, this)}`
+        ? `${await options.additionalData(data, this)}`
         : `${options.additionalData}\n${data}`;
   }