text/template: implement comparison of basic types

Add eq, lt, etc. to allow one to do simple comparisons.
It's basic types only (booleans, integers, unsigned integers,
floats, complex, string) because that's easy, easy to define,
and covers the great majority of useful cases, while leaving
open the possibility of a more sweeping definition later.

{{if eq .X .Y}}X and Y are equal{{else}}X and Y are unequal{{end}}

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/13091045