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.
