Add systemd support

[qarkai]

Hi! Could you provide unit file to start/stop proxy via systemctl?

[terlar]

/etc/systemd/system/pacproxy.service

[Unit]
After=network.target
Description=pacproxy

[Service]
ExecStart=/path/to/bin/pacproxy -c /path/to/proxy-config.pac
KillMode=mixed
Restart=always