go.talks/present: remove presentation logic from data structures

Remove the Template field from Doc, and the Doc field from Section.
The new Render method on Doc wraps the Doc in an anonymous struct that
also contains the template, before passing it to the template iteslf.
The renderElem function wraps Section elements in a similar way.

R=campoy, dsymonds
CC=golang-dev
https://golang.org/cl/6863058
3 files changed