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 I'll pull this out. Doesn't really make too much sense, since we're the ones making the direct API call, not the consumer of our lib. Any information returned from this won't really make sense to an end-user.