[modules] Support promise

lib/module.js

@@ -99,12 +99,15 @@ class Module {
     }
     // Call module with `this` context and pass options
     return new Promise((resolve, reject) => {
-      return module.call(this, options, err => {
+      const result = module.call(this, options, err => {
         if (err) {
           return reject(err)
         }
         resolve(module)
       })
+      if (result && result.then instanceof Function) {
+        return result.then(resolve)
+      }
     })
   }
 }