FirewallD Zones

Hi, I was having trouble defining my installation through other zone. It may be convenient to have a prompt which asks the user if he wants to use the default zone (normally public).

I've [found the line](https://github.com/angristan/wireguard-install/blob/21b5e56eea117ae0c509591a5fa1c9d79b37f383/wireguard-install.sh#L187) which detects if there is a `firewalld` installation. This script does uses the default zone in the `PostUp` and `PostDown` scripts for the peer startup.

I'm leaving this issue here because I am having trouble for 3 days, trying to see if the problem was from the firewall, `wireguard-tools` or some conflict with my docker installation. I'd like to post a pull request as soon as I get some time.

Thanks for this project. It really helped me