html/template: add examples of loading templates from files
html/template: add examples of loading templates from files

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/21854
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

derekmarcotte/go,vsekhar/elastic-go,cloudflare/go,vanloswang/go,ganboing/go-esx,nawawi/go,suncycheng/go,theif519/go,vanloswang/go,codestation/go,ganboing/go-esx,vsekhar/elastic-go,golang/go,josharian/go,cloudflare/go,akutz/go,goodlang/good,golang/go,Cofyc/go,josharian/go,ImJasonH/go,4ad/go,codestation/go,akutz/go,nawawi/go,vanloswang/go,Cofyc/go,vsekhar/elastic-go,derekmarcotte/go,CAFxX/go,cloudflare/go,momchil-velikov/go,vanloswang/go,ImJasonH/go,4ad/go,josharian/go,codestation/go,theif519/go,akutz/go,Cofyc/go,josharian/go,golang/go,4ad/go,goodlang/good,CAFxX/go,momchil-velikov/go,suncycheng/go,cloudflare/go,josharian/go,ganboing/go-esx,nawawi/go,linux-on-ibm-z/go,polyverse-security/go,cloudflare/go,ImJasonH/go,momchil-velikov/go,theif519/go,CAFxX/go,josharian/go,derekmarcotte/go,codestation/go,linux-on-ibm-z/go,Cofyc/go,suncycheng/go,4ad/go,ImJasonH/go,nawawi/go,goodlang/good,polyverse-security/go,CAFxX/go,4ad/go,Cofyc/go,goodlang/good,momchil-velikov/go,4ad/go,polyverse-security/go,derekmarcotte/go,akutz/go,cloudflare/go,theif519/go,codestation/go,josharian/go,theif519/go,nawawi/go,vsekhar/elastic-go,suncycheng/go,polyverse-security/go,goodlang/good,momchil-velikov/go,vanloswang/go,4ad/go,ganboing/go-esx,golang/go,Cofyc/go,linux-on-ibm-z/go,ImJasonH/go,derekmarcotte/go,josharian/go,akutz/go,christopher-henderson/Go,akutz/go,linux-on-ibm-z/go,momchil-velikov/go,vsekhar/elastic-go,CAFxX/go,vsekhar/elastic-go,polyverse-security/go,golang/go,suncycheng/go,ganboing/go-esx,ImJasonH/go,linux-on-ibm-z/go,polyverse-security/go,CAFxX/go,vanloswang/go,ImJasonH/go,golang/go,codestation/go,vsekhar/elastic-go,nawawi/go,linux-on-ibm-z/go,linux-on-ibm-z/go,vanloswang/go,theif519/go,theif519/go,Cofyc/go,suncycheng/go,linux-on-ibm-z/go,vanloswang/go,golang/go,derekmarcotte/go,suncycheng/go,akutz/go,golang/go,codestation/go,codestation/go,suncycheng/go,polyverse-security/go,polyverse-security/go,Cofyc/go,nawawi/go,momchil-velikov/go,4ad/go,goodlang/good,goodlang/good,ganboing/go-esx,ImJasonH/go,cloudflare/go,ganboing/go-esx,ganboing/go-esx,CAFxX/go,derekmarcotte/go,momchil-velikov/go,akutz/go,theif519/go,cloudflare/go,CAFxX/go,nawawi/go,vsekhar/elastic-go,goodlang/good,derekmarcotte/go