storage: createReadStream: accept start/end offsets

RE: #318 

This adds `options.start` and `options.end` to `file.createReadStream`, using the `Range` HTTP header when calling the API.

// @janesconference for a review, since it was all his idea :bulb: :+1:
I think just letting it not provide integrity quietly is fine. RE: stdout, our lib shouldn't ever print to stdout, as it's likely we're going to be used in CLI applications where users of the application that uses our lib shouldn't see warnings/errors from us. If it was a big enough problem, we would throw, but I think the doc suggestion is good enough here.