feature/plural: expose low-level plural matcher

Oftentimes the plural form can be exposed without
computing the intermediate number representation.
Allow this.

Change-Id: I6b8819bd175bd5ab1a7e9a9395448fa5c9b4274b
Reviewed-on: https://go-review.googlesource.com/58430
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>