When requesting fields in the search, get, update, etc APIs, we should be able to control where those values come from with these prefixes:

• _source: extract the field value from the _source field
• _fields: retrieve a stored field value
• _doc: retrieve the value from fielddata

The _doc option is particularly useful for retrieving dates as, no matter the format they're specified in in the source, they will be accessible as milliseconds-since-the-epoch from fielddata, without the need to allow dynamic scripts.

While on the subject, perhaps we should support this same syntax in scripting, instead of docs['foo']?