Add show_label option to add the possibility to hide the label (code 128 only).

In some cases it is not desirable to have the label embbeded in the image, as
it may be displayed separately for better control over the text.
Merge pull request #17 from ecstaticpeon/master

Add show_label option to add the possibility to hide the label (code 128 only).

Thanks @ecstaticpeon!