Index-less json parsing

[HLWeil]

#95

Json writers now contain a noNumbering flag, which skips indexes when writing:

open FsSpreadsheet
open FsSpreadsheet.Net

let wb = new FsWorkbook()
let ws = FsWorkbook.initWorksheet "MySheet" wb
let r1 = ws.Row(1)
r1.[1].SetValueAs "A1"
r1.[2].SetValueAs "B1"
let r2 = ws.Row(2)
r2.[1].SetValueAs "A2"
r2.[2].SetValueAs "B2"

wb.ToRowsJsonString(noNumbering = true)

->

{
  "sheets": [
    {
      "name": "MySheet",
      "rows": [
        {
          "cells": [
            {
              "value": "A1"
            },
            {
              "value": "B1"
            }
          ]
        },
        {
          "cells": [
            {
              "value": "A2"
            },
            {
              "value": "B2"
            }
          ]
        }
      ]
    }
  ]
}