add @ref support in json data

[bsorrentino]

add support #ref(<symbol id>) as valid json value. Reference implies that there will be a reference section

[
{
  "name": "references",
  "rows": [
     { "id": "note left", "value": "note left /' of p1 '/", "additional": ["this note is displayed left", "end note"]},
  ]
},
{
  "name": "sequence",
  "rows": [
    { "id": "note", "value": "#ref(note left)"}
  ]
}
]