Add default scopes for values if enabled

Something as easy as an options hash for `time_for_a_boolean` method. 

`scopes: enabled` could generate:

```
scope :deleted, ->{ where.not(deleted_at: nil) }
scope :not_deleted, ->{ where.(deleted_at: nil) }
```

Would adding this benefit this project? Forked it and curious if this is worth putting a PR.
