Fix #990: ascii reference implementation

[gwilherm]

Example:

@startuml
participant Alice
actor Bob

ref over Alice, Bob : init

Alice -> Bob : hello

ref over Bob
  This can be on
  several lines
end ref
@enduml

Gives:

                       ┌─┐         
                       ║"│         
                       └┬┘         
                       ┌┼┐         
     ┌─────┐            │          
     │Alice│           ┌┴┐         
     └──┬──┘           Bob         
╔══════╤╪═══════════════╪══════╗   
║ REF  ││               │      ║   
╟──────┘│               │      ║   
║ init  │               │      ║   
╚═══════╪═══════════════╪══════╝   
        │    hello      │          
        │──────────────>│          
        │               │          
        │        ╔══════╤═════════╗
        │        ║ REF  │         ║
        │        ╟──────┘         ║
        │        ║ This can be on ║
        │        ║ several lines  ║
     ┌──┴──┐     ╚════════════════╝
     │Alice│           ┌─┐         
     └─────┘           ║"│         
                       └┬┘         
                       ┌┼┐         
                        │          
                       ┌┴┐         

[gwilherm]

Just noticed that final Bob lost his name..

Edit:
That actually also happens with notes : https://www.plantuml.com/plantuml/txt/SoWkIImgAStDuKfCBialKd3AJ-BYoiilILN8Bqk5S1M083mZjIXL8RDO8RTI8IynDrN1dYirBmMaZii1o03g1m00