font BBC tag should allow all valid CSS font-family strings.

For dealing with sceditor copy-paste font-family lists more gracefully.
Example: paste text from another web page, with "font-family: 'Segoe UI', Tahoma, sans-serif;"
http://www.simplemachines.org/community/index.php?topic=524219.msg3711839#msg3711839

In response to #1768

Signed-off-by: Angelina Belle <angelinabelle1@hotmail.com>