Add support for templating in response bodies

[deerbone]

I have added support for using text/template engine in the response body.

This change allows users to declare dynamic responses using templating from path parameters, query parameters and in a limited way the request body.

If a template response uses the request body, the request currently needs to be a JSON and contain the "Content-Type": "application/json" header.

[deerbone]

I feel like this code is now ready for round two. Re-requesting review.

[deerbone]

@joanlopez just bumping this in case you missed it. No hard feelings if you don't have the time.