Add autoincrementing relationship label

[daniel-simpson]

Definitely more of a nice to have, but I've found myself labelling flows through certain systems (especially those with async / callback mechanisms) by manually prefixing labels with 1., 2., etc.

PR on the way to add this with a preprocessor: AUTONUMBER_RELATIONSHIPS()

[daniel-simpson]

Oops, I also just realised that I used my custom colour scheme in the example. Let me know if that's a dealbreaker and I can update accordingly

[daniel-simpson]

For anyone else viewing this, it is already implemented (in a more robust fashion) in C4_Dynamic.puml