tour: add a tour of text/template and html/template

[Sajmani]

The Go text/template and html/template are widely used and well-documented, but a tour would provide a better way of teaching the template language, starting from the basics and moving on to more advanced features. I'd also like to see non-HTML uses of text/template called out, such as generating markdown or complex SQL queries.

[minux]

The current tour is currently very light on the coverage of the standard library. I think a whole new section for the standard library is warranted.

[rakyll]

/cc @campoy @adg