Feature request: support automatic scope generation

ActiveRecord's `enum` is capable of automatically generating scopes. Would be nice to have support for the feature here too.

For instance:
```rb
class Item < Sequel::Model
  plugin :enum
  enum :condition, [:mint, :good, :poor]
end
```

```rb
Item.mint
```
equivalent to 
```rb
Item.where(condition: 0)
```