Revise AddThisBlock implementation to take advantage of D8 Plugins

[doylejd]

D7 version of AddThis implemented a block that allowed users to chose a field formatter to render it. The block used the default field settings to control the rendering.

D8 implements blocks as plugins which gives more flexibility in its configuration. It would allow for more flexibility if we allow the block to be fully configurable instead of depending on a field formatter for the configuration and rendering.

@TODO

• Expand the block configuration to allow for additional configuration.
• Update render method (Currently always renders addthis_basic_toolbox using the field formatter default settings)

[doylejd]

PR for this is up here: https://github.com/d8-contrib-modules/addthis/pull/12