Add support for gutenberg metabox registration support parameters #1191

With Gutenberg around the corner, it would be great to be able to take advantage of the custom parameters introduced with Gutenberg: https://wordpress.org/gutenberg/handbook/extensibility/meta-box/

This would allow devs to mark metaboxes as incompatible, compatible, or a fallback.

If I have time at some point in the next month or so I'll make a PR, but it shouldn't be much work so if someone else gets to the faster, great!