0.4.0

Backward incompatible changes:

* The _format_json function now requires nested calls to live inside
  quotes. See `ok.sh help _format_json` for a usage example.

Misc additions and fixes:

* Fix a regex bug with mawk (thanks, dubiouscript!).
* Fix several script breakages in a Busybox environment.
* Add a Docker file for quick local development and testing.
* Add several commands to the Makefile to streamline development.
* Update hardcoded VERSION value in the script.
* Add more information to CONTRIBUTORS doc.