Add simple support for iterator-based event subscription

[kgutwin]

I really like this library and it worked great on my first try! Thanks for doing the work of tracing and prototyping this to make it easy to get a good data feed out of the cloud.

As I integrated it into my standard monitoring system, I debated how often to call the get_grill_status() method, and as I read the code in detail, I realized it would actually be pretty easy to convert the status polling into a thread-based subscription method. With this approach, no delay is needed in the polling loop; the MQTT topic controls how frequently the client is updated.

Using this approach from the client side is even easier than polling:

t = traeger.traeger(username, password)

for status in t.grill_status_subscription():
    print(json.dumps(status))

The only downside to using this method is that you don't get to control how frequently the updates come, so if (as in your example script) you want a slower or more consistent update frequency, using the existing get_grill_status() method is better.