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:
Should make a note that if either of these is set, data integrity will be turned off, even if they specify it. Not sure if we should do anything more than that for them e.g. stdout a warning if they explicitly set the `validation` option as well?