Rationale: This hook will allow someone to overwrite the get_username_string function
Input arguments: $mode, $user_id, $username, $username_colour, $guest_username, $custom_profile_url (get_username_string clone)
Output format: string
Output semantics: Returns what get_username_string returns.
Infrastructure: This would allow people to generate things such as username prefixes, pretty viewprofile urls, different viewprofile urls, and other custom effects on the username as it is displayed.