Change default SecurePassword column name

Currently the ActiveModel::SecurePassword module force to create a requires you to have a password_digest attribute. This patch provide the option to switch the attribute name

```ruby
class Visitor
  extend ActiveModel::Callbacks
  include ActiveModel::Validations
  include ActiveModel::SecurePassword

  define_model_callbacks :create

  has_secure_password(validations: false, column_name: :password)

  attr_accessor :password, :password_confirmation
end
```