whitelists and blacklists can be global

Until now, `Feature.whitelist` and `.blacklist` took a required first argument -- the name of the feature:

```ruby
Arturo::Feature.whitelist(:foo) do |recipient|
  recipient.plan.has_foo?
end
```

Now whitelists and blacklists can be global. The block takes the feature as the first argument:

```ruby
Arturo::Feature.whitelist do |feature, recipient|
  recipient.plan.has?(feature.to_sym)
end
```

The former is now implemented on top of the latter.