Refactor AddThis block to allow for extensibility

[doylejd]

In the Drupal 7 version of AddThis the block can be configured based on any FieldFormatter that is defined for AddThis. With the changes in D8, I was not able to reproduce this functionality.

@todo:

• Investigate the best way to make the block extensible by future contrib/custom modules.
• Can we use the FieldFormatters in the same way that D7 did?
• Is there a better option to handle the Field/Block displays to make them extensible by external modules? (Ie. Controllers, ??)