go.tools/cmd/present: track page views for every slide.
go.tools/cmd/present: track page views for every slide.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/124720043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools