feature/plural: add Select

Selects message based on plural form of
argument.

Change-Id: I8730e12b7b3c08317cdca3f899eebb8f23b2d83a
Reviewed-on: https://go-review.googlesource.com/58551
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
4 files changed