Generate file in source, then add to copy queue

I don't think generating files directly into the `_site` directory is the best way to go about this type of a plugin. It's better to generate the files in the source then add them to the queue so Jekyll copies them over.

You might want to edit a bit more as I just made only the edits required to make this work, but it seems to be less of a fight with how Jekyll is designed. I also was never able to get my destination to work as a subdirectory until making this change.