Field notation should be able to find fields in maps other than map[string]interface{}

[djaglowski]

There's probably an integration test that should be added at some point, but the issue is really shared by all input/parser/transformer operators. For now, I think it's enough that we're validating the output is map[string]interface{}, but we should come up with a better way to just handle other types of maps too, and then test that it works across the board.

Originally posted by @djaglowski in #485 (comment)