Warn user if z's datafile path is a directory.

Cause this means z will fill up the directory with temp files, and not
work.
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
Merge branch 'master' into dev