Accept a lambda for context in factory

Like `decorates_association`, you can now pass a context lambda to `decorates_assigned`, which receives the current controller instance and returns a hash:

```ruby
class ArticleController < ApplicationController
  decorates_assigned :article, context: ->(controller){ {user: controller.current_user} }
end
```

The actual calling of the lambda has been extracted to the Factory·

Closes #467