Allow files to be preprocessed before inclusion.

This builds upon the functionality in #1, adding the ability to run a file's content through a preprocessor before being added to the JSON. My use case I'm building a backend-less website, where the data is simply defined in json, and I'd like to be able to easily include the content of files in non-JSON formats (e.g. markdown) in my json file.

If there's anything you'd like me to do to make these PRs better, let me know.