Append, rather than prepend to PROMPT_COMMAND
Replace an echo with a redirect
Handle empty PROMPT_COMMANDS
remove mktemp dependency

Closes #103
Merge branch 'master' into prompt_command_append

Conflicts:
	z.sh
Indentation is 4 spaces.

When I was a feckless youth, I preferred single space indentation for
shell. Now in my wise old age that feels cramped. Don't even say
anything about tabs - I will slap you.
Cleanup

Rename variables, improve comments, etc, for clarity and to better fit
my programmerly preferences.
comments
Alternate approach to appending to PROMPT_COMMAND

Seems to work well no matter what weirdness is already in
PROMPT_COMMAND.

Thanks @godlygeek. Closes #99
Merge branch 'master' into cleanup
Merge branch 'master' into cleanup
Merge branch 'cleanup' of github.com:rupa/z into cleanup
redirect to dev/null a bit more POSIXly
Merge branch 'master' into dev