Provide markup for core/post-author block

[goldenapples]

This plugin doesn't filter the output of the core/post-author block, making it not usable out of the box in block themes.

It would be good to provide some rendered output for this block, following roughly the markup of the core authors block, and document how to extend or filter this as desired.

[johnbillion]

Related: #58

[johnbillion]

This needs to apply to the core/avatar and core/post-author-name blocks too