PlantUML JSON support

[ldeluigi]

Please add support for json: https://plantuml.com/json

Maybe with ::json::

::end-json::

And

```plantjson
{
...
}

[mikitex70]

Hi @ldeluigi, have you tried to use @startjson and @endjson in the code diagram, inside the fence? It should work...

[ldeluigi]

I tried putting an !include Value.puml directive

Value.puml contains @startjson and @endjson

And it doesn't work:

It gives syntax error at line 3

At line 3 I have the first open curly bracket of the json

[mikitex70]

Without includes it works. Example:

# Title

```uml
@startjson
{
   "fruit":"Apple",
   "size":"Large",
   "color":"Red"
}
@endjson

Output:
![immagine](https://user-images.githubusercontent.com/2070527/119018702-a89f9700-b99c-11eb-8081-676bb5afe2cf.png)

[mikitex70]

It works with include also. Example (test.md):

# Title

```uml
@startjson
!include value.json
@endjson

value.json:
```json
{
   "fruit":"Apple",
   "size":"Large",
   "color":"Red"
}

output:

[ldeluigi]

Curious: I'll try more use cases and I will let you know

[ldeluigi]

[ldeluigi]

I didnt' put the !include inside two @startjson @endjson But now I think I need to do it

[ldeluigi]

Ok now it works! Well thanks for the support