Change the api of Transformers.

[havardh]

With the discussion in https://github.com/havardh/workflow/pull/139#discussion_r216442751 we would like to consider to change the API of transformers to explicitly return values for noop instead of the current implicit return falsy.

We must however take it into consideration that it might be a valid response from a transformer to cull subtrees by returning either null or undefined.