Add route_info lookup

[chrismccord]

@josevalim any input on how we should partially make %Phoenix.Router.Route{} public? Still needs docs about what is and isn't private, but not sure what's best at the moduledoc/or router_info function doc level. Would appreciate your thoughts

[josevalim]

I think all of the current fields are safe to expose, so it is probably a matter of making the module public and hiding the documentation for all other functions (or perhaps move those functions to another module, so we don't end-up @doc false everything).