Enable/Disable real HTTP request

As default, if you do not mock a host, a real HTTP request will do, but sometimes you should not permit real HTTP request, so...

For disable real http request.

```js
nock.disableNetConnect();
```

So, if you try to request any host not 'nocked', it will thrown an NetConnectNotAllowedError.

```js
nock.disableNetConnect();
http.get('http://google.com/');
// this code throw NetConnectNotAllowedError with message:
// Nock: Not allow net connect for "google.com:80"
```

For enabled real HTTP requests.

```js
nock.enableNetConnect();
```