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:
Not used anywhere in our code. It would just be an extra bit of information that developers would have. Nothing was being returned before this PR, but before the integrity PR, I believe this response object (which is straight from `request`) was being returned.