Angled arrows to denote time passing

[davidje13]

As discussed for another library: bramp/js-sequence-diagrams#129

It would be desirable to be able to show messages taking time to reach their destination, allowing other messages to get mixed up with them. Maybe something like:

A -> ...my arrow ID: Delayed message # Message is sent
B -> C: something else happens
...my arrow ID -> B                  # Message arrives

Need to consider text overlap, e.g.:

A -> ...id1: Message 1
B -> ...id2: Message 2
...id1 -> B
...id2 -> A

[davidje13]

This has been implemented in a basic form using the proposed syntax (except the message is defined on the second part, not the first). This also works with self-communications.

Text overlap is an issue, but for now it will have to be up to the user to make sure text is displayed nicely; I can't think of a way to enforce a good layout.