Add special renderers for metadata #62

this seems to only have one use-case and utterly outside the scope of this project, i'd suggest making a new lib layered on top of the viewer to achieve this.

There is a growing interest for machine learning in the prismarine community. It is an other use case for prismarine-viewer, but it is not at all outside the scope of the project. We add features, then the user are not forced to use them.

Now, we already agreed in other discussions that prismarine-viewer should be split in multiple modules, but that's an orthogonal discussion and not very relevant here.