Add flag to force bagpipe formatting

This addresses issue #1. The command line switch '-p' will force the rendering code to treat grace notes and note stems as if the key were a type of bagpipe, even if it is not (e.g. K:D).

From a code review standpoint, I'm not proud of this patch. It feels dirty to have a command line switch to do this formatting. I still think the right way to do this is to provide an ABC formatting directive, but this gets the job done right now so I can print tunes for my band.