Mechanism to pass complex types through simpler transformers

[etrain]

Currently transformers are maps from A => B, but if I have some complex type like (Int, Image) I can't pass it into an Image => DenseVector[Double] Transformer and expect it to do the right thing. We can probably come up with some implicit magic that allows us to apply Transformers to objects like this.

Similar to #249 , this came up in the context of data augmentation in a discussion with @shivaram today.