Allow reading only n tail bytes from a file

[stephenplusplus]

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.

[ryanseys]

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.

[stephenplusplus]

How about just allowing:

file.createReadStream({ end: -10 })

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

[jgeewax]

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.

[stephenplusplus]

Cool cool. Let's try it out and if we find the community hates it, we can try again!