Update field_example to demonstrate configurable field formatter plugins

Problem/Motivation

It would be nice to have the field_example module demonstrate how a formatter plugin can expose user definable configuration on the manage display page.

Proposed resolution

Update one of the two existing formatter plugins so that it provides a configuration option. Right now they're both pretty simple formatters and we can make one of the two more complex without detracting from the learning experience.

This updates the \Drupal\field_example\Plugin\Field\FieldFormatter\ColorBackgroudFormatter plugin so that it has a user defined configuration option which can toggle on/off some additional logic that will attempt to automatically set the foreground color of the text depending on the lightness of the background color. This seemed like a real use-case without requiring a lot of extra code unrelated to thing we're trying to demonstrate which is how to make a formatter configurable.