Add _value method to enumerized class

I want to use it in using a hash as `:in` option.
I'm not good at English, so i wrote a example code :sweat_smile:

```ruby
class User
  extend Enumerize

  enumerize :role, in: {:user => 1, :admin => 2}
end

user = User.new.tap{ |u| u.role = "user" }
user.role_value
# => 1
```