All Inclusive: Emphasize usage of the router, pass req/res to snippet, implement snippet parameters.

This PR is a pretty epic combination of features for vain.

* As of now the vain router is the **preferred** way of integrating vain into your application. It still works as a normal layout engine, but if you use it that way you won't get request and response data in your snippets.
* Snippets now support parameters. See changes to README for information on usage.
* `responseMiddleware` is gone for good.