Add support for GitLab style plantuml blocks and additional diagram types

[th-yoo]

• Support for GitLab style ```plantuml blocks
• Extends the diagram types available to include {ebnf, regex, ditaa, gantt, chronology, chen}.

Examples

• PlantUML tags are used in the same way as before

    @startuml
    A <|-- B
    @enduml

• Defaults to @startuml

    ```plantuml
    A <|-- B
    ```

    ```plantuml
    @startuml
    A <|-- B
    @enduml
    ```

   @startuml
   A <|-- B
   @enduml

All three blocks above are equivalent. However, blocks without plantuml code tag (triple backtick + "plantuml") will not be rendered in Gitlab.

• Support for newly added tags

@startditaa
+---------+  /--------\   +-------+
| cBLU    +--+cAAA    +---+Version|
|         |  |  Data  |   |   V3  |
|    +----+  |  Base  |   |cRED{d}|
|    |cPNK|  |     {s}|   +-------+
|    |    |  \---+----/
+----+----+    
@endditaa

• Some PlantUML tags have their options

@startditaa -E
+---------+  /--------\   +-------+
| cBLU    +--+cAAA    +---+Version|
|         |  |  Data  |   |   V3  |
|    +----+  |  Base  |   |cRED{d}|
|    |cPNK|  |     {s}|   +-------+
|    |    |  \---+----/
+----+----+    
@endditaa

[guillaume-d]

Couldn't this be implemented using Gollum 6's custom language handlers instead, or am I missing something?

Also the @start... syntax could stay without support for new diagram types. Not adding them there may be both good (backward-compatibility) and bad (inconsistent with backtick syntax).

[dometto]

Thank you for submitting this @th-yoo!

Couldn't this be implemented using Gollum 6's custom language handlers instead, or am I missing something?

Thanks for mentioning this @guillaume-d -- I think that would indeed be the best and most maintainable way to implement this functionality.

@th-yoo do you think you could rework this PR to incorporate this idea? Let me know if you need pointers to implement the language handler. It might (but also might not) make sense to implement this in gollum instead of gollum-lib, compare the Mermaid implementation here.