text/template: add 'nil' as a keyword in the language

text/template: add 'nil' as a keyword in the language
The keyword reprents an untyped nil and is useful for
passing nil values to methods and functions. The
nil will be promoted to the appropriate type when
used; if a type cannot be assigned, an error results.

R=rsc, dsymonds
CC=golang-dev
https://golang.org/cl/6459056