Adds examples showing loading templates from files and
executing them.

Shows examples:
- Using ParseGlob.
- Using ParseFiles.
- Using helper functions to share and use templates
in different contexts by adding them to an existing
bundle of templates.
- Using a group of driver templates with distinct sets
of helper templates.

Almost all of the code was directly copied from text/template.

Fixes #8500

Change-Id: Ic3d91d5232afc5a1cd2d8cd3d9a5f3b754c64225
Reviewed-on: https://go-review.googlesource.com/21854Reviewed-by: Andrew Gerrand <adg@golang.org>