internal/format/plural: added plural package

This package will be moved out of internal at some point.

In the proposal (12750) this package is slated to be a
subpackage of feature. In practice format/feature/plural
makes more sense. But as there is no need for the feature
as of yet (or ever), we put it directly under format.

Change-Id: I8d5c9be00d458087680662dc6c117eee2dcfc566
Reviewed-on: https://go-review.googlesource.com/19333
Reviewed-by: Nigel Tao <nigeltao@golang.org>

internal/format/plural/plural.go

@@ -0,0 +1,38 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package plural defines the grammatical plural feature.
+//
+// The definitions in this package are based on the plural rule handling defined
+// in CLDR. See
+// http://unicode.org/reports/tr35/tr35-numbers.html#Language_Plural_Rules for
+// details.
+package plural
+
+import "golang.org/x/text/internal/format"
+
+// Form defines a plural form. The meaning of plural forms, as well as which
+// forms are supported, vary per language. Each language must at least support
+// the form "other".
+type Form byte
+
+const (
+	Other Form = iota
+	Zero
+	One
+	Two
+	Few
+	Many
+)
+
+// Interface is implemented by values that have a plural feature.
+type Interface interface {
+	// PluralForm reports the plural form of a value, depending on the
+	// language declared by the given state.
+	PluralForm(s format.State) Form
+}
+
+// TODO
+// - Select function
+// - Definition for message package.