Renamed agenda filetype from dotoo_agenda to dotooagenda

* VIM's filetype loading format allows filetype_name format so
  dotoo_agenda matched dotoo filetype as well.
Fixed headline.save()

* Earlier I was calling setline(), which overwrote the headlines
  content, however if the content was longer / shorter this would result
  in a bad overwrite leaving / removing content outside of the
  headlines. Logging the last_lnum into headline so I can delete the
  headline content and then add the result of serialize() using
  append(). That way there are no conflicts and I affect only the
  content of the current headline which I am saving and nothing else.