fix 'multiple repeat' error from regex for c++ filetypes

unescaped ++ is invalid regex causing easytag to fall back to the
vimscript highlighting which does not split the regex into small chunks
that vim can handle.
Merge pull request #63: fix 'multiple repeat' error from regex for c++ filetypes

See pull request #63 on GitHub:
  https://github.com/xolox/vim-easytags/pull/63