Show application URL when it starts

Then one execute without specifying -listen, it prints:

Lightmeter ControlCenter is running on http://[::]:8080

And when -listen localhost:8080 is used:

Lightmeter ControlCenter is running on http://127.0.0.1:9090

When -listen 0.0.0.0:9090 is used:

Lightmeter ControlCenter is running on http://[::]:9090

[::] is the zero IPv6 address, being the equivalent to "0.0.0.0" in IPv4.

Funnily enough, it's a clickable link in most terminals :-)

Ref #161 (closed)