This allows having links from one slide to another one in the same presentation.
Also it is needed in the new tour to cross reference lessons.
R=r
CC=golang-codereviews
https://golang.org/cl/46130043
Godoc depends on go.talks/pkg/present by way of go.tools/pkg/blog.
Better to keep all godoc dependencies in one place.
R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/13656047