Ability to choose between A, B or C modes for code_128

[diogocsc]

Hi, it would be nice to be able to choose the 128 mode.

BARBY works fine with A, but barby (smallcaps) doesn't.

My suggestion is to add a new option code128... so we could write it like this
has_barcode :barcode,
outputter: svg,
type: :code_128,
code128: 'B',
value: Proc.new { |p| "#{p.number}" }

is has_barcode.rb then we could do something like
@@barcode_configurations[args.first].barcode_class.new(options[:value].call(self), options[:code128])