template: implement multi-word variable instantation for formatters.

template: implement multi-word variable instantation for formatters.
Before one could say
	{field}
or
	{field|formatter}
Now one can also say
	{field1 field2 field3}
or
	{field1 field2 field3|formatter}
and the fields are passed as successive arguments to the formatter,
analogous to fmt.Print.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/3385041