code: function getImmediateCustomParentTag(tag) {
    var ptag = tag
    while (!getTag(ptag.root)) {
      if (!ptag.parent) break
      ptag = ptag.parent
    }
    return ptag
  }
docs: Loop backward all the parents tree to detect the first custom parent tag
  @param   { Object } tag - a Tag instance
  @returns { Object } the instance of the first custom parent tag found