Automatically create missing parent directories in _Z_DATA path

[silenc3r]

When parent directories of _Z_DATA don't exist we should automatically create them.

[ericbn]

Could also touch the file if it does not exist. This would remove the need for changes like https://github.com/rupa/z/commit/78f0d97 and tests like

# no file yet
[ -f "$datafile" ] || return