[WIP] Argument type validation with user-friendly error messages

`type_` is analagous to werkzeug's [routing converters](http://werkzeug.pocoo.org/docs/0.9/routing/#builtin-converters); they convert input values to ensure that the view function receives arguments of the correct type.

If you only want to do type-checking of the raw input values, you should use the `validate` argument instead.