Consider adding EditorBlockCustomization class to project.

[jmichaelward]

During an internal OOPS-WP/Gutenberg call this morning as part of 5 for the Future, we discussed the use case for providing style modifications to native Gutenberg blocks, both within the editor itself and on the WordPress front-end (e.g., engineers may want to modify the presentation of, say, the Header block and want a mechanism for registering that customization).

This use case is separate from #20, because the responsibility of the EditorBlock class is to provide a uniform way to register new, custom blocks with WordPress. It is possible, however, that both the EditorBlock and a new EditorBlockCustomization object might both implement an interface for registering block styles. Therefore, I'm opening this ticket to capture discussion around that idea, and to consider proposals for moving forward with this approach.