Make the invoice number formattable

[mamhoff]

In order to satisfy jurisdictions where invoice numbers have to conform specific formal requirements (number of digits, prefix, whatever) you can now define a Spree::PrintInvoice::NumberFormattter class which initializes with a integer, and implements a to_s method returning a formatted number.

[tvdeyen]

Could you please add a note about this feature into the README?

😎👌👍🏼