function highest(arr) {
  var res = arr.slice().sort(function(a, b) {
    return a.lvl - b.lvl;
  });
  if (res && res.length) {
    return res[0].lvl;
  }
  return 0;
}
Get the highest heading level in the array, so
we can un-indent the proper number of levels.

@param {Array} `arr` Array of tokens
@return {Number} Highest level