Get the same integration with FreeBSD pkg as with other package managers.

Basic working plugin available at: <https://github.com/radhus/etckeeper/commit/f849bcc558c2c83b8738e31d2aee56d979c39555>

> I've merged this. Of course it would be good to get the main Makefile
> able to install it. Please open new todos if you have updates and thanks!
> [[done]]
> --[[Joey]]