Arguments for url helper

[botanicus]

url(:post, @post, format: "foo")
    - @post ... /post/:slug => @post.slug
    - @post ... /post/:id => @post.id

[botanicus]

The interface has to be the same for all the routers

[botanicus]

It should be able to works with namespaces:

url("admin:post", @post)

or work with current class or something

[reactormonk]

Bonus if it works as
url(@post)
if it's a resource
see usher issue