Allow escape for period in config nested keys

[PEMapModder]

Expected result:

{
  "a": {
    "b.c": "b period c",
    "b": {
      "c": "b slash c"
    }
  }
}

$config = new Config("abc.json", Config::JSON);
var_dump($config->getNested('a.b\.c')); // string(10) "b period c"
var_dump($config->getNested("a.b.c")); // string(9) "b slash c"

Note: this mechanism isn't fully tested.

[jasonw4331]

@SOF3 I think this can be used in pmmp's version