New operator to match flags stored in hstore

Could not find a different solution than to create a special operator for this use case. Feedback is very welcomed.
Yes we can try also this option, if you meant:
```ruby
if_attribute :"permissions_flags.testing" =>  has_flag { true }
```

But, IMO, it reads strange, maybe:
```ruby
if_attribute :"permissions_flags.testing" =>  is_set { true }
```