New option: -x

Removes the current directory from the data file.

Use case: sometimes `z tmp` lead me to `/tmp` whereas I was expecting to
be in `~/tmp`, with the new option I can do `z -x tmp`: it removes
`/tmp` from the data file and jumps to `~/tmp`.
Merge pull request #94 from baskerville/prune-pwd

New option: -x
Fix a few typos
Regenerate README from man page

Closes #97
Merge pull request #98 from baskerville/fix-typo

Fix a few typos
Throw away tmp files if mv fails

Attempt to get rid of stale temp files that occasionally drop. While
there is anecdotal evidence that this doesn't solve the problem, it
doesn't hurt, and it's the only place I can see to fix it.
revert warning if z.sh is being exec'd and not sourced

Because we can't reliably determine if we're really being exec'd.
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