Modularize metadata preprocessor code

I think this functionality may ultimately make more sense within Solidus proper, but I'd like to test the ideas out separately for now, hence this module. The README documents the goals and current implementation. I'm planning to add tests next week and complete RDF support (currently it only accepts `og:type`).
You're always re-using the same crossroads instance, adding the same routes again and again when your pre-processor is run. Move this line to the exported function below, and you can even remove the `ignoreState` line, since the router instance will be used only once.