Support template vars in req body + memoize nested response body fields + memoize request body fields

[kkajla12]

1. Adds support for template vars in req body:

{
            "name": "myTest",
            "request": {
                "method": "POST",
                "url": "/my-endpoint",
                "body": {
                    "blah": "{{ anotherTest.val }}"
                }
            },
            "expectedResponse": {
                "statusCode": 200,
                "body": {
                    "blah": "{{ anotherTest.val }}"
                }
            }
}

2. Adds support for nested memoization
3. Adds support for memoizing request body fields

{
            "name": "myTest",
            "request": {
                "method": "POST",
                "url": "/my-endpoint",
                "body": {
                    "hello": "world"
                }
            },
            "expectedResponse": {
                "statusCode": 200,
                "body": {
                    "blah": "{{ myTest.request.body.val }}"
                }
            }
}

Closes #8
Closes #36