Generate unknown types on the fly.

## Example

### Setup
```ruby
class Person
  include TypeCollection::Base
end

class SoberPerson < Person; end
```

### Previous behavior
```
irb> Person.all_types
=> [SoberPerson]
irb> Person.get_associated_type("Sober")
=> SoberPerson
irb> Person.get_associated_type("Drunk")
TypeCollection::UnknownChildType: Unregistered type: Drunk
```

### Newly augmented behavior
```
irb> TypeCollection.create_unknown_types = true
irb> Person.get_associated_type("Drunk")
=> DrunkPerson
irb> Person.all_types
=> [SoberPerson, DrunkPerson]
```