mp-weixin: Fix Component don't inherit props from Vue.extend(). Support externalClasses option.

Fix inheritance issue by using Vue.options instead of Vue.extendOptions.
Add externalClasses to wechat MP component options.

src/platforms/mp-weixin/runtime/wrapper/component-base-parser.js

@@ -20,6 +20,7 @@ export default function parseBaseComponent (vueComponentOptions, {
   initRelation
 } = {}) {
   let [VueComponent, vueOptions] = initVueComponent(Vue, vueComponentOptions)
+  vueOptions = VueComponent.options || vueOptions
 
   const options = {
     multipleSlots: true,
@@ -96,6 +97,10 @@ export default function parseBaseComponent (vueComponentOptions, {
       __e: handleEvent
     }
   }
+  // externalClasses
+  if (vueOptions.externalClasses) {
+    componentOptions.externalClasses = vueOptions.externalClasses
+  }
 
   if (Array.isArray(vueOptions.wxsCallMethods)) {
     vueOptions.wxsCallMethods.forEach(callMethod => {