Adds input accessory for string and text row types. With working tests.

Not quite sure how to test the done_action property. This ought to do for now though.

Example usage:

```ruby
{
  title: "Title Goes Here",
  type: :string,
  value: "Something",
  input_accessory: :done,
  done_action: -> { puts "Done Action" }
}
```

works for string rows, text rows, and all others that inherit from those :)