Feature: add json-support to languages

As upcoming features I'm going to offer as pull requests extensively use JSON files, I thought it might be a good idea to have the possibility of including them in the generated output for documentation purposes. Sadly Douglas Crockford [removed comments from the initial JSON-design](https://plus.google.com/118095276221607585885/posts/RK8qyGVaGSr). Therefore (valid) JSON-files can only be included as plain code.

I decided to implement a new `codeOnly`-flag complementary to the existing `commentOnly`-flag, and use this for JSON-files. As an alternative we could just add `'*.json'` to the existing Javascript entry, but I still have the idea to find a better solution to document JSON-files, therefore I started with this simple and separated addition, without touching the existing Javascript-entry.

Not to forget to mention, that I added the `.groc.json` and `package.json` files to the project-configuration, in order to test and document the implementation.

Hope you like that solution. :cold_sweat: 

Cheers,
Stephan