Support rendering multiple output files for a given input file

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As of #51 , we have implemented basic 1:1 generation of HTML for Noosphere content. However, some kinds of content may be best served over the web when rendered with multiple output artifacts. An example of this would be a video file that also would benefit from a companion HTML wrapper that brings in a nice control UX.

So, we should support possibly rendering multiple outputs for a given input in our transformers.