Feature: Implement "extended" logging using a marker character

If a log message contains a magic character, everything after that marker
will be considered "extended" information. Log targets default to showing
this extended information but qb_log_ctl() may be used to indicate that
the extended information should be stripped instead.

The use case is a log message that has some information of value to administrators
and other information that is only of value for debugging. Two targets can be
configured such that the administrator log is not cluttered with debug information,
but a separate debug log is still available with complete information.

The magic character is the bell (\a). If someone wants to log an actual bell
character, they can do so in the extended information or via a %c format
(only the first bell is magic and only in the format string).