Support retries for NetworkErrors

[tsantef]

Now that the NetworkError error type is implemented in v4 It would be nice to add a retry adaptor using the https://www.npmjs.com/package/backoff library or similar. It would be configured by passing in a retry option object in the constructor. Defaults to off. The options could include an array of errors types to retry on (defaults to [NetworkError]), initial delay, and max delay.