Add preview_email tool

See thread "Preview LUCI Notify emails" on luci-announce for context.

Change-Id: Iee7c864790bb99217e39c962099504f3d34802ac
diff --git a/cipd.ensure b/cipd.ensure
index 0f377f4..7e3deb2 100644
--- a/cipd.ensure
+++ b/cipd.ensure
@@ -41,5 +41,8 @@
 # vpython
 infra/tools/luci/vpython/${platform} git_revision:a3fffc0930f82e7653250c3ae68c2642f3db61d0
 
+# preview_email
+infra/tools/preview_email/${platform}  git_revision:f884439683c62e9b5a9dd627faf74c042cf77f06
+
 # YAPF
 infra/tools/yapf git_revision:a3fffc0930f82e7653250c3ae68c2642f3db61d0