fix: pkg: compatibility layer with BSD systems re-worked. (fixes #10)

Use of ``/bin/sh`` instead of ``/bin/bash``. Removed all bashism.
Using BSD version of ``sed`` and ``date`` when available.
fix: fail with error message when config path exists but is not a file. (fixes #11)

For example, the config file could be a directory.
fix: pkg: would create a directory ``gitchangelog.reference`` which would break a lot of things.
fix: dev: remove a whitespace !minor