Review the URL schemes

[samoht]

Currently, --datakit and --listen arguments of the GitHub bridge, as well as the -a address of the GH webhook listener. For instance we have:

• tcp://localhost:5640 (--listen)
• tcp:localhost:5640 (--datakit)
• [localhost]:5640 (-a)

We should try to converge to something more consistent as it already caused me much confusion.

See #188