no support for MSI (Plessey) barcodes

[petaflot]

lib does not support https://en.wikipedia.org/wiki/MSI_Barcode

specification is open, AFAIK there are no license restrictions

[petaflot]

suggested fix : https://github.com/WhyNotHugo/python-barcode/pull/191

fails some tests, however I am not familiar at all with those so some assistance would be appreciated

generated barcode was tested successfully with Metrologic / Honeywell MS9520 scanner.

[rmkane]

Left a comment on the PR in regards to variations of MSI.