Add “boolean” scopes to date and datetime columns

Sometimes, people use `date` and `datetime` columns as a boolean field.

For example, [paranoia](https://github.com/radar/paranoia) has a `deleted_at` `datetime` column that stores the timestamp of when a record was destroyed. If the field is blank, the record is not destroyed. If the field contains a date, the record is considered as destroyed and we know the exact timestamp of when it happened.

This pull request adds two additional scopes for each `date` and `datetime` column that returns records according to the pattern described above.