fix: should export the pitch function

fixes #1677

TODO:
Need to add tests later

Author: haoqunjiang

src/pitcher.ts

@@ -24,7 +24,7 @@ const pitcher: webpack.loader.Loader = code => code
 
 // This pitching loader is responsible for intercepting all vue block requests
 // and transform it into appropriate requests.
-pitcher.pitch = function() {
+export const pitch = function() {
   const context = this as webpack.loader.LoaderContext
   const rawLoaders = context.loaders.filter(isNotPitcher)
   let loaders = rawLoaders