Handle SIGTERM by saving world and shutting down gracefully

[chrisjoj]

This is an initial attempt at a way to handle restarts and shutdowns gracefully as a resolution for #4

It does this by sending the 'exit' command to the TerrariaServer stdin which saves the world and shuts down the server.
It was based on https://github.com/alexivkin/terraria-server-container/blob/master/buildcontext/serverwatcher.sh

I've tested this with vanilla-1.4.2.1. If this looks like a good approach, I can do some testing with TShock to see if a similar approach would work for those images.

[meonkeys]

console does not exist. Should that be /vanilla/console?

[meonkeys]

Nice, thanks! Works for me after fixing the one console typo on line 60. Some feedback:

1. I'd appreciate some comments around the more esoteric Bash code (maybe what is esoteric Bash is just convention to a more experienced Bash programmer).
2. After applying this patch I'm unable to attach to the container and issue commands to the server. We could maybe clean it up with more (named) pipes.

I could maintain this with great comments and a good deal of study or if it were in Python.