number: add plural integration

also added consideration to use only
VisibleDigits interface instead of
plural.Interface.

Change-Id: Ibb2a9a0eddc3f6fe7689b7fa3268f2bf0c8a97dc
Reviewed-on: https://go-review.googlesource.com/60771
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
9 files changed