docs: function(exts, fn, options) {
        if (arguments.length === 1 && typeof exts === 'string') {
          return this.getEngine(exts);
        }
        exts = utils.arrayify(exts);
        var len = exts.length;
        while (len--) {
          var ext = exts[len];
          if (ext && ext[0] !== '.') ext = '.' + ext;
          this._.engines.setEngine(ext, fn, options);
        }
        return this;
      }