Add option to output debug info in org-mode format

[drdv]

Resolves #26 and partially addresses #28.

Now we can define

help: SCR := /path/to/makefile-doc.awk
help: ## Show this help
	@awk -v $(DEBUG) -f $(SCR) $(MAKEFILE_LIST)

and use make DEBUG=1 whenever debug info is needed. The debug info is in org-mode format.

Unit tests will be added in a separate PR.