Nicer schema for string literals + string

[domoritz]

I believe this only requires minor changes in https://github.com/vega/ts-json-schema-generator/blob/master/src/TypeFormatter/EnumTypeFormatter.ts.

export interface MyObject {
    bar: "ok" | "fail" | "abort" | string;
}

produces

> [email protected] run /Users/domoritz/Developer/UW/ts-json-schema-generator
> ts-node ts-json-schema-generator.ts "--path" "test.ts" "--type" "MyObject"

{
  "$ref": "#/definitions/MyObject",
  "$schema": "http://json-schema.org/draft-04/schema#",
  "definitions": {
    "MyObject": {
      "additionalProperties": false,
      "properties": {
        "bar": {
          "anyOf": [
            {
              "enum": [
                "ok"
              ],
              "type": "string"
            },
            {
              "enum": [
                "fail"
              ],
              "type": "string"
            },
            {
              "enum": [
                "abort"
              ],
              "type": "string"
            },
            {
              "type": "string"
            }
          ]
        }
      },
      "required": [
        "bar"
      ],
      "type": "object"
    }
  }
}

The enums could be consolidated.