message: link in plural package by default

This makes plural messages work out of the box for
generated catalogs.

Change-Id: Ifc7e457d7bf909dcf067f749366e8a49ac31f6b6
Reviewed-on: https://go-review.googlesource.com/81836
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

message/message.go

diff --git a/message/message.go b/message/message.go
index 841a98a..a3473bf 100644
--- a/message/message.go
+++ b/message/message.go
@@ -8,6 +8,9 @@
 	"io"
 	"os"
 
+	// Include features to facilitate generated catalogs.
+	_ "golang.org/x/text/feature/plural"
+
 	"golang.org/x/text/internal/number"
 	"golang.org/x/text/language"
 	"golang.org/x/text/message/catalog"