Some people uses the current timestamp for the DNS serial number. The
current implementation detects timestamps between September 9 2001 and
March 17 2030. Implementing it has reduced the date based serial
detection to a range between 1900 and 2999, however it should not be a
problem at all because DNS didn't existed before 1900 and, if it still
exists in 3000, there is literally almost 1000 years to fix the problem.
Some people does not use the common YYYYMMDDXX format and/or does not
want to use the "; serial" comment right after the serial number. This
commit allow multiple formats to be defined, hense more users can use
the plugin. A huge consequence of this is that user can also define
additional custom formats depending on their needs.
Some users may want to explicitly update the DNS serial number. Instead
of forcing them to update on each write, a new configuration option is
used so they can chose turn it off.
