Create example for AsyncProgressWorker & AsyncProgressQueueWorker #164

There is currently on example for these progress workers. The documentation is kind of a bit confusing here: https://github.com/nodejs/node-addon-api/blob/master/doc/async_worker_variants.md

It doesn't really explain how to use the progress callback in the OnProgress function...just randomly makes a Callback().Call({Env().Null(), Env().Null(), Number::New(Env(), *data)}); call with two Env().Null()'s with no explanation as to what this is doing. I guess by passing two nulls it forces this to the Progress callback in javascript...but it's all a bit confusing.

Would be nice to get a proper example on here put together for these progressworkers. In the nan library you have to specify each the callback and the progresscallback to the worker, but it doesn't seem to be the case in the new node-addon-api, example/test can be found here: https://github.com/nodejs/nan/blob/e222068f9d99967897a8cefa6c505534b36116ad/test/cpp/asyncprogressworker.cpp

I am porting a project from nan -> node-addon-api currently, and feel an example on how the progress callbacks work would be beneficial to me and others also.

Thanks.