allow inline nested property to be inherited from superclass using new :inherited option

This PR allows nested property objects to be inherited from superclass and enhanced.  The primary motivation for this was to allow subclasses to effectively re-open a nested property of the superclass and add validations.  A contrived example:

```ruby
class RegistrationForm < Reform::Form
  property :contact do
    property :first_name
    property :last_name
    property :email
    validates :email, :presence => true
  end
end

class UpdateRegistrationForm < RegistrationForm
  property :contact, :inherit => true do
    property :previous_email, :virtual => true
    validates :previous_email, :presence => true
  end
end
```

Instances of UpdateRegistrationForm will now have a contact property consisting of all properties and validations defined in both RegistrationForm and UpdateRegistrationForm.

Note that right now I've added a new :inherit option which must be set to `true` for the subclass to inherit the property from the parent class.  By default :inherit is assumed to be false, so the behavior is not affected implicitly.  If this PR is considered useful, I am open to using a different naming strategy.

Note: I realize that the property method actually comes from the representable gem, so if it's more appropriate to create a patch there instead I can try that.