Added option for multiple authors

Added an option to enable or disable support for multiple post authors.
Changed input ID

Changed input ID to match input name.
Initial code

Initial code for class. Non-working as of this commit.
Update nickname in addition to display name

When saving post author data to the user profile, update the nickname
with the same value as the display name.
Don't use post author info on Author archive pages

If browsing an Author archive page, don’t use post author data.
Added option for multiple authors

Added option for multiple authors, and enable by default.
Adjusted spacing

Adjusted spacing for better readability.