Added :url locator to get url of current page

[milanito]

Description

This PR is supposed to solve #64 by adding a :url locator. it can be used from a :follow or the root page

I have also added a rescue on the follow page to not crash when a follow is not working (404 mostly)

Here is an example of usage

data config[:list_selectors][:link], :follow do
    title 'css=h1'
    url :url
  end
end

[PedroAugustoRamalhoDuarte]

Good feature @milanito thanks for that!!