Add option to enable/disable logging per call

It would be convenient to turn on or off logging (especially for debugging) with something like:
```js
fetchJson.enableLogger();
fetchJson.get(url).then(handleData);                  //call is logged
fetchJson.get(url, { log: false }).then(handleData);  //call is NOT logged
```

See the two instances of `if (fetchJson.logger)` in **fetch-json.ts**.