transformer functions should be able to return an array

[jipperinbham]

There's been many cases where a single document needs to either be split up or contains a nested array that need to be sent down the pipeline.

The current transformer function only supports returning a single document but could be altered to also support returning multiple documents.

This would address https://github.com/compose/transporter/issues/139 and https://github.com/compose/transporter/issues/140