Nzbget package

This is a change that adds the ability to update the installed nzbget binaries when the container runs.

I use it to run a build of the current nzbget 13.0 testing release. Because there are no prebuilt packages (that I could find) I had to build it myself and then find a way of including it in the docker container.

What I came up with is inspired by some of your other containers. 

I've included my build of nzbget in the new packages directory. I've also modified firstrun.sh to accept an environment variable (NZBGET_PACKAGE) which specifies the name of the package to install. When run it looks in /config/packages for a file with the specified name and if found it extracts it and overwrites the nzbget 12 binaries that are installed by default in the container.

It does that purely by overwriting the already installed files - no fancy package management shenanigans or anything like that.

My build of nzbget also contains a fix for a bug that I found (whilst experimenting with this solution) and have reported to the nzbget maintainer.

I'm sure there is a much more elegant solution but, for me, this works and is quite simple.