Add visualization debug option to the docca build

I am hoping to eventually get a visualization debug option working on the docca build using [xslt-visualizer](https://github.com/evanlenz/xslt-visualizer) (which itself has some pending optimizations that will affect the output, so it's not quite ready yet). [Here's a basic demo](http://xmlportfolio.com/xslt-visualizer-demo/). [Here is some documentation](https://github.com/evanlenz/beast/blob/saxon-xsl/doc_visualized/README.md) that provides an overview in relation to docca. And [here's a sample output](http://xmlportfolio.com/forVinnie/libs/beast/doc_visualized/html/beast/ref/boost__beast__buffer_bytes.html) for Beast from a while back. The "doc_visualized" build was using bash scripts and got removed from the docca project. I would eventually like to make it bjam-friendly and get it back into docca as a build option. Once I get the latest xslt-visualizer changes checked in, we should be good to explore that.