Allow inline blocks to extend concrete classes

[khamusa]

Implements #19

With this change, users are able to reuse concrete classes as sections while also customizing their behavior inline, enabling advanced inheritance-like behaviors. Example:

class EmailAddressesSection < Tabasco::Section
  # ...
  def has_successful_notification_message?
    has_content!(success_notification)
  end
end

section :mailing_address, EmailAddressesSection do
  def success_notification
    "Mailing address updated successfully"
  end
end

section :living_address, EmailAddressesSection do
  def success_notification
    "Living address updated successfully"
  end
end

# ...

expect(form.mailing_address).to have_success_notification_message
expect(form.living_address).to have_success_notification_message

Implementation details

Given we don't want the original concrete class to be redefined, we subclass it before evaluating the inline block on its subclass. This was quite a fit given the DSL was already subclassing Section.

However, getting this to work properly required class instance variables that support the DSL configuration to be inherited by subclasses.

After experimenting with a variety of approaches, I decided to pull active support's class_attribute extension. It's a shame that this brings in a bunch of other dependencies, but for now this seemed like the best solution. We can revisit this later (maybe even copying over class_attribute's implementation given it's MIT-licensed).

I also had to figure out with the confusion around test_ids when using a concrete class (this was already a known logged issue). I decided the concrete class test id will never be used, unless when the class is being explicitly .loaded. This decision felt simply natural when using the framework. It feels consistent and foster code reuse.