text/template: allow {{else if ... }} to simplify if chains

text/template: allow {{else if ... }} to simplify if chains
The method is simple: the parser just parses

        {{if A}}a{{else if B}}b{{end}}

to the same tree that would be produced by

        {{if A}}a{{else}}{{if B}}b{{end}}{{end}}

Thus no changes are required in text/template itself
or in html/template, only in text/template/parse.

Fixes #6085

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13327043