Based on work by Russ Cox. From his CL:

        This is generally useful but especially helpful when trying
        to use the built-in boolean operators.  It lets you write:

        {{if not (f 1)}} foo {{end}}
        {{if and (f 1) (g 2)}} bar {{end}}
        {{if or (f 1) (g 2)}} quux {{end}}

        instead of

        {{if f 1 | not}} foo {{end}}
        {{if f 1}}{{if g 2}} bar {{end}}{{end}}
        {{$do := 0}}{{if f 1}}{{$do := 1}}{{else if g 2}}{{$do := 1}}{{end}}{{if $do}} quux {{end}}

The result can be a bit LISPy but the benefit in expressiveness and readability
for such a small change justifies it.

I believe no changes are required to html/template.

Fixes #3276.

R=golang-dev, adg, rogpeppe, minux.ma
CC=golang-dev
https://golang.org/cl/6482056