code: function buildTemplate(filename, options, callback) {
    var isAsync = callback && typeof callback === 'function',
        getTemplateContentFn = options.getTemplate  && typeof options.getTemplate === 'function' ? options.getTemplate : getTemplateContentFromFile;
  
    // sync
    if (!isAsync) {
      return builtTemplateFromString(
          getTemplateContentFn(filename, options),
          filename,
          options
      );
    }
  
    // function to call when retrieved template content
    function done(err, templateText) {
      callback(err, builtTemplateFromString(templateText, filename, options));
    }
  
    getTemplateContentFn(filename, options, done);
  }
docs: Builds a template
  If callback is passed, it will be called asynchronously.
  @param {String} filename The path or the name to the template
  @param {Object} options The options sent by express
  @param {Function} callback (Optional) The async node style callback