Changed script to be more standardized and dynamic

[Aadniz]

I came across this bash script, and I felt an urge to maybe improve it a bit.

• In the main loop, I use an array instead of 3 variables.
• I moved the gateway adding above the loop, since it just needs to be ran once.
• You can now set variables via environment variables instead of putting it inside the script. This might be useful for running it as a systemd service for example.
• Added a trap to delete created IPv6 when application is terminated.

Since there is so much changed, feel free to comment and change on what you want. I didn't implement /48 yet.