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