Initial value for multidimensional arrays. Fixes #68 in joshfire/jsonform.

Fixed support for multi-dimensional arrays initial value. The
following JSON should populate initial values correctly.

{
  "schema": {
    "arr": {
      "type": "array",
      "items": {
        "type": "array",
        "items": {
          "type": "string"
        }
      }
    }
  },
  "value": {
    "arr": [
      ["Item 1", "Item 2"],
      ["Item A", "Item B"]
    ]
  }
}