Make Mojolicious::Validator supports named routes

At the moment, Mojolicious::Validator only preload parameters from GET's url or POST's body. It will not load the parameters from a named route by default. $validation->input has to be called explicitly for named route parameters.

This commit lets Mojolicious::Validator preload the parameters from named routes as well.

I have amended the corresponding documentation and added one test case in validation_lite_app.t.
You also need tests for precedence (GET/POST parameters before placeholders), and the exclusion of reserved stash values.