feat: use es2015 syntax to re-export default export (#32)

BREAKING CHANGE:

This changes the output generated by create-index and can effect CI if it runs linting tests against create-index.

src/utilities/createIndexCode.js

@@ -14,7 +14,7 @@ const buildExportBlock = (files) => {
   let importBlock;
 
   importBlock = _.map(files, (fileName) => {
-    return 'export ' + safeVariableName(fileName) + ' from \'./' + fileName + '\';';
+    return 'export { default as ' + safeVariableName(fileName) + ' } from \'./' + fileName + '\';';
   });
 
   importBlock = importBlock.join('\n');