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 commit 'b9afd5faaddd62067a5f37084c35476e8b2e4cc8'

Conflicts:
	z.1
	z.sh

knu-z uses _z_cmd --del for implmenting this.
Null merge commit '19de191e257788b3aeb7eb76b32bb1f87291d0d6'

knu-z already has these typos fixed.
Null merge commit 'db827eec154212ca828d267d25a64984ad8358ac'

knu-z already takes care of this.
Quit if unexpectedly executed or sourced, like when PS1 is undefined.
Null merge commit '8aac576b7cfcc2ac4fbad7e298bcacc29a03b731'

knu-z now checks for PS1.
Change the separator from a semicolon to a newline in PROMPT_COMMAND.
Null merge commit '54be46728d67673ebeb609d901988cfea58828a6'

knu-z still prepends a command to PROMPT_COMMAND.
Null merge commit '7570b48b168ad93b702daa470d79afd708cd418c'

knu-z doesn't use echo & grep but glob matching.
Null merge commit '9c6a7aca3e6870909018d3de83527d04d3c5f37d'

knu-z already takes care of empty PROMPT_COMMAND.
Do not fail if $1 is "(".
Make mktemp(1) optional.