While using exp/template in practice, the syntax for
indexing values using the "index" action was found to be
very inconvenient for frequent use when handling dynamic
data from maps such as the ones used with json and yaml,
that use a type like map[string]interface{}.

For these kinds of maps, the default handling of fields as
{{.Field}} makes the task of handling the several references
significantly more pleasant and elegant, and is equivalent
to what's currently done in the "template" package and in
other external packages (e.g. mustache).

Even with this change, the index action is still relevant
as it allows indexing maps in other scenarios where keys
wouldn't be valid field names.

R=golang-dev, r, gustavo
CC=golang-dev
https://golang.org/cl/4898043