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.
Even if this plugin is small, DNS zone editing may not be very frequent.
Therefore, there is no point defining those functions every time vim
start. Moving them to the autoload directory solve this problem.
