Dividers

[davidje13]

Seen on http://plantuml.com/sequence-diagram,

Add support for full-width horizontal dividers with optional messages;

A -> B
---
A -> B
--- divide message
B -> A

The same component could also be used for delays (represented by dotted agent lines):

A -> B
...
A -> B
... delay message
B -> A

And gaps of arbitrary size:

A -> B
gap 10
A -> B
gap 15: gap message
B -> A

[davidje13]

Consider how ... will interact with proposed syntax in #41 - which should change?

[davidje13]

This has been implemented with a new divider keyword to keep the --- and ... syntax options available for other features. Lines, gaps, delays and tears are all supported, as well as configurable heights.