Fix: Can't use Unix Sockets in Quick Tunnel mode

[rhyswilliamsza]

From https://github.com/cloudflare/cloudflared/issues/1294:

When you try to use a unix socket (--unix-socket) without a domain configured, cloudflared still asks for the --url parameter.

This PR aims to address the above by adding unix-socket as an accepted flag for spawning quick tunnels. I unfortunately couldn't find an elegant way to mock out the RunQuickTunnel command, so I had to add a small work around. Please let me know if there is a better standard for this.

Thanks!