Initial customization of the_author() tag
Keep the space after the ampersand
Add input fields for editing the_author() customization
Prefer Co-Authors-Plus for guest author support

The Co-Authors-Plus plugin enables us to use much richer metadata for
authors, via the "Guest Authors" function, which also enables us to
provide more useful schema.org markup.

Fall back to the old customized the_author() tag if the Co-Authors-Plus
plugin is not activated or if we're rendering posts prior to using
Co-Authors-Plus (currently hardcoded as a test against post ID 9000).

Signed-off-by: Dan Scott <dan@coffeecode.net>