Allow reading only n tail bytes from a file

Carrying on from #415, a user can specify a start and end range to read from a remote file. The JSON API also allows reading n tail bytes from a file, e.g. range=-100 would be the last 100 bytes. Our API currently looks like:

file.createReadStream({ start: 0, end: 10 })

We could allow specifying a range in another way:

file.createReadStream({ range: "0-10" })

Which would open the doors for:

file.createReadStream({ range: "-100" })

Definitely open to other ideas of how we can support ranges of both types: "start and end" and "end offset".

Note: start and end was added to match the native fs module's createReadStream's arguments, which behaves the same was currently as ours.

Another idea I'm not too attached to but figured I'd throw it out. What about:

file.createReadStream({ tail: 10 })

This would override or be always overridden by start/end. Problems include introducing another parameter, parameters that void other parameters, but benefits are it's clear in its purpose.

How about just allowing:

file.createReadStream({ end: -10 })

and adding documentation to explain that's reading -10 tail bytes?

I'm 👍 to end: -10 (the same way in CSS you can do right: -10px for example). Definitely would rather this than adding a new parameter... But you guys are the experts.