Add support for dashed message lines

[kjagiello]

Also adds support for omnidirectional message arrows. Demo of both features below.

Input

ali->bob: "Hello!"
ali<--bob: "Hello back!"

Output

┌──────┐         ┌──────┐ 
│ ali  │         │ bob  │ 
└──────┘         └──────┘ 
    │     Hello!     │    
    │───────────────▶│    
    │                │    
    │  Hello back!   │    
    │◀---------------│    
    │                │    
┌──────┐         ┌──────┐ 
│ ali  │         │ bob  │ 
└──────┘         └──────┘