function parseLinkTitle(state, pos) {
  var code,
      start = pos,
      max = state.posMax,
      marker = state.src.charCodeAt(pos);

  if (marker !== 0x22 /* " */ && marker !== 0x27 /* ' */ && marker !== 0x28 /* ( */) { return false; }

  pos++;

  // if opening marker is "(", switch it to closing marker ")"
  if (marker === 0x28) { marker = 0x29; }

  while (pos < max) {
    code = state.src.charCodeAt(pos);
    if (code === marker) {
      state.pos = pos + 1;
      state.linkContent = unescapeMd(state.src.slice(start + 1, pos));
      return true;
    }
    if (code === 0x5C /* \ */ && pos + 1 < max) {
      pos += 2;
      continue;
    }

    pos++;
  }

  return false;
}
Parse link title

  - on success it returns a string and updates state.pos;
  - on failure it returns null

@param  {Object} state
@param  {Number} pos
@api private