code: function (exec) {
        var _exec = ["pandoc"],
            conf = this.configs;
  
        _exec.push("-S");
        _exec.push("-o " + conf.target);
        _exec.push("--epub-metadata="   + conf.metadata);
        _exec.push("--epub-stylesheet=" + conf.stylesheet);
        _exec.push("--epub-cover-image=" + conf.cover);
  
        _exec.push(this.files["chapters"].join(" "));
  
        exec.push(_exec.join(" "));
  
        return exec;
      }
docs: publishEPUB
  Setup Pandoc Option (Markdown files to EPUB)
  @param exec {array} exec strings.
  @returns {array} Execute Command Strings