Ignore generated doc/tags directory
Remove helptags regeneration, fixes #1
Merge pull request #4 from kepi/helptags

Do not call helptags in ~/.vim/doc, might not exist

When using a vim package manager such as Vundle or Pathogen, the path
~/.vim/doc might not exist. Since package managers take care of
generating help tags when installing a new package, remove the
unnecessary call.