fix: require of plugin that exports ES module

Transpiled ES modules don't export the default export directly as
CommonJS modules, they always export an object and the default export is
under the key "default" in this object.

Author: jirutka

src/cli.js

@@ -60,8 +60,10 @@ const read = file => new Promise(resolve => fs.readFile(file, 'utf8', (error, da
   resolve(data);
 }));
 
+const interopRequire = object => object && object.__esModule ? object.default : object;
+
 const getPlugins = config => Object.keys(config.plugins || {})
-  .map(plugin => require(plugin)(config.plugins[plugin]));
+  .map(plugin => interopRequire(require(plugin))(config.plugins[plugin]));
 
 const config = cfgResolve(cli);