Add optional _last_modified metadata to JSON

[ttu]

Option to add last modified timestamp for each object.

{
  "user": [
    {
      "id": 1,
      "name": "James",
      "age": 40,
      "location": "NY",
      "work": {
        "name": "ACME",
        "location": "NY"
      },
      "_last_modified": "2017-10-14T09:45:10"
    },
    {
      "id": 2,
      "name": "Phil",
      "age": 25,
      "location": "London",
      "work": {
        "name": "Box Company",
        "location": "London"
      },
      "_last_modified":  "2017-10-15T12:12:12"
    }

Typed models need to have property defined

public DateTime _last_modified { get; set; }

Feature branch: https://github.com/ttu/json-flatfile-datastore/tree/add-last-modified