In the course of discussions on a number of issues (see, for example, #248, #10, #30 and #227), the topic of extensions to the cookiecutter.json format has come up. Rather than have the discussion scattered across multiple threads, let's come up with a next-generation config format that is flexible enough to cater for all the various use cases.

Some thoughts:

• We need to continue parsing the current format as "version 1.0" for backward compatibility.
• Do we stick with JSON, or should we use YAML? (An advantage of using YAML is that the config version is detectable from the filename, an advantage of sticking with JSON is that we don't have 2 parsers in the code).
• Maybe include a config version, so future enhancements can be versioned more easily?