Create interface to add messages for new classes

[rossellhayes]

Add an interface for adding methods to hinted_class_message_list() and friendly_class_message_list() from outside tblcheck.

Possible solutions:

• Check for additional message formats in options().
• Convert message generators into R7 generics.
  • This would involve refactoring the class problem to hold the entire object and expected objects, not just their class and length.