Allow "make" to take a block

[wulftone]

I'm finding that making a new blueprint for a simple little thing isn't always the nicest way to go about doing things, so some flexibility in how make is used would be nice. MyModel.create allows you to supply a block to futz with attributes, and this functionality seems like a perfect fit for make.

User.make :admin, do |u|
  u.password = 'xxxxxx' unless Rails.env.production?
end

etc..