docs: function mixin(src, dest, skipEmpty) {
      // TODO: refactor into common module
      var dest = dest || {}, src = src || {},
          keys = Object.keys(src), len = keys.length, key;
  
      for (var i = 0; i < len; i++) {
          key = keys[i];
          if (!exists(dest[key]) && (skipEmpty && exists(src[key]))) {
              dest[key] = src[key];
          }
      }
      return dest;
  }