Add a canonical option to page helper

[gabriel-cardoso]

We'd like to write something like that

page :product, resource: product
page :product_in_category, resource: product, path: product_category_path(category, path), canonical: product_path(product)

This should add a meta tag <link rel="canonical" href="#{ product_path(product) }" /> to both pages.

When option canonical is set, we should check that the given page really exists in the skeleton to avoid pointing on a 404

[gabriel-cardoso]

Canonical URL's should be absolute