Add `indexable` & `PM::Table::Indexable`

* Instead of having to implement `table_data_index`, this will get the
first letter from each group automatically.

207 specifications (358 requirements), 0 failures, 0 errors
Merge pull request #207 from clearsightstudio/table_data_index_class_method

Add `indexable` & `PM::Table::Indexable`