Standalone metadata preprocessor

@caolan, I would like to use your metadata preprocessor, but I would like to use it with the [marked](https://github.com/chjj/marked) engine. Are you interested in splitting it out into a standalone repo? Or would you like me to?

FWIW, I wrote a number of other pre-processors that I'm using with the marked engine. My current plan is to adopt the same preprocessor API that you are using (taking and returning an object with a `markdown` property).