Trigger BufWritePre/BufWritePost autocmd events in xolox#notes#save()

It's hard to estimate the consequences of this change, but there's really
only one way to find out, hence I'm committing the change. The previous
behavior was a bug anyway, it's just that it could have been hiding a lot
of other bugs and incompatibilities until now... I'm keeping my fingers
crossed ;-)

See also issue #88 on GitHub:
  https://github.com/xolox/vim-notes/issues/88