Improve serialization

[sdorra]

At the time of writing, content-collections serialize the generated documents as JSON. The problem with this approach is that JSON has a limited set of data types (string, number, boolean, null, object, or array), and this is why the generated documents have only those data types as well (#21).

It would be nice if data types such as Date could be used as part of the document as well. We could possibly achieve this by using a library such as serialize-javascript instead of JSON.stringify.