code: function markedpp (src, options, callback) {
    if (typeof options === 'function') {
      callback = options
      options = null
    }
  
    options = Object.assign({}, defaults, options || {})
  
    Lexer.lex(markedpp.ppInclude, src, options, function (err, tokens) {
      let out = tokens
      if (!err && tokens) {
        out = Parser.parse(tokens, options)
      }
      callback(err, out)
    })
  }
docs: Markdown Preprocessor
  
  @module markedpp
  @param {String} src - markdown source to preprocess
  @param {Object} options - options
  @param {String} options.dirname - dirname of markdown source file - required to include other files
  @param {Function} callback - callback function
  ```
  function({Error} err, {String} data)
  ```