Improve LaTeX cleanup automcommand

To explain these changes:
  * BufDelete will trigger this not just when vim closes but when the
    file closes too
  * For an autocommand you get the current file using <afile>, not %
  * :silent is used to suppress the "press Enter to continue" message
    which I don't want to receive every time I close a tex buffer
Improve rules for buffer hiding

Now buffer hiding is set on a per-buffer basis and is only set for
buffers whose files are under a sion/ directory, meaning it will be
triggered for real files in my home directory, and not for files higher
up (which need to be handled carefully) or scratch files and other made
up things which need to be destroyed when hidden so that they don't
leave empty buffers lying about.