feat($medium-zoom): support selector option.

Author: ulivz

packages/@vuepress/plugin-medium-zoom/index.js

@@ -1,5 +1,8 @@
 const { path } = require('@vuepress/shared-utils')
 
-module.exports = {
+module.exports = (options, context) => ({
+  define: {
+    SELECTOR: options.selector || '.content img'
+  },
   clientRootMixin: path.resolve(__dirname, 'mixin.js')
-}
+})

packages/@vuepress/plugin-medium-zoom/mixin.js

@@ -1,10 +1,12 @@
+/* global SELECTOR */
+
 import './style.css'
 import zoom from 'medium-zoom'
 
 export default {
   mounted () {
     setTimeout(() => {
-      zoom('.content img')
+      zoom(SELECTOR)
     }, 1000)
   }
 }