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

* Instead of having to implement `table_data_index`, this will get the
first letter from each group automatically.
* Backwards compatible -- will defer to `table_data_index` if you have implemented that.

207 specifications (358 requirements), 0 failures, 0 errors

```ruby
class MyTable < PM::TableScreen
  indexable

  def table_data
    [{
      title: "Apple",
      cells: []
    }, {
      title: "Google",
      cells: [],
    }, {
      title: "Microsoft",
      cells: []
    }]
  end
end
```