render(options, cb) {
      return this._request('POST', '/markdown', options, cb, true);
   }
Render html from Markdown text.
@see https://developer.github.com/v3/markdown/#render-an-arbitrary-markdown-document
@param {Object} options - conversion options
@param {string} [options.text] - the markdown text to convert
@param {string} [options.mode=markdown] - can be either `markdown` or `gfm`
@param {string} [options.context] - repository name if mode is gfm
@param {Requestable.callback} [cb] - will receive the converted html
@return {Promise} - the promise for the http request