This commit puts the client creation logic into a function.
By running the script followed by `add_client`, the user will be asked a
few questions and a new peer will be added.
Removing client is **not** supported.
The client files have random names.
Fix for https://github.com/angristan/wireguard-install/issues/3
without resolvconf package, systemd unit file will fail in Debian 10 and ubuntu 18.04 or higher versions
see error below
```
Oct 17 12:11:39 Asus-VivoBook wg-quick[25620]: [#] ip link add wg0 type wireguard
Oct 17 12:11:39 Asus-VivoBook wg-quick[25620]: [#] wg setconf wg0 /dev/fd/63
Oct 17 12:11:39 Asus-VivoBook wg-quick[25620]: [#] ip -4 address add 10.9.0.3/24 dev wg0
Oct 17 12:11:39 Asus-VivoBook wg-quick[25620]: [#] ip link set mtu 1420 up dev wg0
Oct 17 12:11:39 Asus-VivoBook wg-quick[25620]: [#] resolvconf -a wg0 -m 0 -x
Oct 17 12:11:39 Asus-VivoBook wg-quick[25620]: /usr/bin/wg-quick: line 31: resolvconf: command not found
Oct 17 12:11:39 Asus-VivoBook wg-quick[25620]: [#] ip link delete dev wg0
Oct 17 12:11:39 Asus-VivoBook systemd[1]: wg-quick@wg0.service: Main process exited
```
more info here : https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=930735