auto_barcode takes prefix option
Add message prefixes

Prefixes can be used to provide a method for reducing the chance of conflicts
with other, non-related, barcodes on pages. The prefix option can be used when
autobarcoding pages so that each barcoded message begins with the provided
prefix. Then when decoding barcoded messages we can also provide a prefix. When
decoding, the prefix is used to filter out messages that do not begin with the
prefix, thereby ignoring any other barcodes that may be present on the page.