fix: tree.walk api

Author: cossssmin

lib/index.js

@@ -2,7 +2,9 @@ const srcset = require('srcset')
 const postcss = require('postcss')
 const merge = require('lodash.merge')
 const isUrl = require('is-url-superb')
+const {walk} = require('posthtml/lib/api')
 const postcssBaseUrl = require('./plugins/postcss-baseurl.js')
+
 const defaultTags = {
   a: {
     href: true,
@@ -67,6 +69,8 @@ module.exports = (options = {}) => tree => {
   options.inlineCss = options.inlineCss || false
   options.tags = options.allTags ? merge(defaultTags, options.tags) : options.tags || {}
 
+  tree.walk = tree.walk || walk
+
   const process = node => {
     // Skip if `url` was not provided
     if (options.url && (typeof options.url !== 'string' || options.url === '')) {