Allow writing index.html, data.js without pushing to gh-pages

On our gh-pages branch we have a few different things, docs, other benchmarks, etc, and I want finer-grain control over how things are pushed to the branch. I think it makes sense to have an option that outputs these assets (edit: i.e. as html+js files, in a particular directory, without uploading them to git) to the user to let them do what they want with them.