Caller: allow early return when job incomplete

I'm using Textract in a web application.

I'm enqueuing jobs using `extractor.start_document_analysis` and storing the returned job ID in a database.

Later, I call `extractor.get_result(job_id)` to get the response for processing.

At the moemnt, `get_result` calls `t_call.get_full_json` and that kicks off a while loop that blocks until the job is in some way finished (e.g `job_status != "IN_PROGRESS"`).

I'd really like a `blocking=False` flag that will simply retun the job status if it's `IN_PROGRESS`. This is beacuse I'm already polling for completeness in my application and I don't want to block my polling workers in this loop.

Of course, I can use the boto3 API to check the status directly before calling `get_result`, but it would be nice to have this as a feature built in.

I'm happy to work on a PR if this seems like a useful feature.