Add `Interval.of(duration, end)` to compliment `Interval.of(start, duration)`

[rtyley]

This adds a new Interval.of(duration, end) way of obtaining an Interval - the mirror of the existing method:

Interval.of(start, duration)

...but for easily getting an Interval that ends at a known instant, with a known duration!

This kind of interval is quite often handy in realtime analytics ("give me the last 30 minutes of results").

[jodastephen]

Surprised this wasn't there! thanks