Update posthtml-parser require

Align the posthtml-parser require with a change introduced to posthtml-parser: posthtml/posthtml-parser@f8ae42d#diff-a2a171449d862fe29692ce031981047d7ab755ae7f84c707aef80701b3ea0c80L210-L211

Author: azolotko

lib/index.js

@@ -3,7 +3,7 @@
 const fs = require('fs');
 const path = require('path');
 const posthtml = require('posthtml');
-const parser = require('posthtml-parser').default;
+const parser = require('posthtml-parser').parser;
 const {match} = require('posthtml/lib/api');
 const expressions = require('posthtml-expressions');