[feat]: extend core

[Scrum]

• add source

  this.version = pkg.version
  this.name = pkg.name
  this.plugins = typeof plugins === 'function' ? [plugins] : plugins || []
  this.source = ''

   options = options || {}
+  this.source = tree

   if (options.parser) parser = options.parser
   if (options.render) render = options.render

   tree = options.skipParse ? tree : parser(tree, options)

   tree.options = options
   tree.processor = this