Delete dokuwiki files on close

I only edit dokuwiki files from Firefox's It'sAllText plugin. When
editing sections on the same page, It'sAllText gets confused and loads
the previously edited section from cache, replacing the current
section's content.

Deleting the cached file when closing the buffer solves this. Now you
don't need to remember to run !rm % before quitting, but you still have
to remember to save *before* you quit so that It'sAllText can load the
file content first.
Only set shellslash when it's available
Make tex-related settings local to the buffer instead of global
Put TeX cleanup autocommand in an augroup so it won't duplicate

Following best practices from Learn Vimscript the Hard Way, I've put
this in an augroup so that vim can replace it every time it's sourced
instead of duplicating the autocommand.