Other: Updated `webpack` to version 4.

BREAKING CHANGE: CKEditor 5 environment was updated to use `webpack@4`. `webpack@4` introduced major changes in its configuration and plugin system. CKEditor 5 tools and build configuration were updated to work with `webpack@4` now and will not work with `webpack@3.

Feature: Added the table feature to the build. Closes #12.

BREAKING CHANGE: The license under which CKEditor 5 is released has been changed from a triple GPL, LGPL and MPL license to a GPL2+ only. See ckeditor/ckeditor5#991 for more information.