Change variables to be overwritten by environment variables

Instead of setting variables using `foo = "bar"` style, a alternative could be to use `foo = "${foo:-"bar"}"` since it would allow defining variables beforehand.

This is very useful if you want to use scripts from here in Docker since it's often best to only change application data (game folders, logs etc.) and not binaries/scripts.

It would not be required change the variables in the function scripts. There could be potential problems regarding existing variables on the system, but I have not found any on a new Ubuntu 16.04 (using `printenv`). Another problem I see is integrating it with all the branches and other ongoing development without breaking stuff.

Thoughts?