Add option to enable/disable logging per call

[dpilafian]

It would be convenient to turn on or off logging (especially for debugging) with something like:

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.