Discard unassigned reads

[jvolkening]

This is a simple patch which provides the option to discard unbinned reads during demuxing rather than putting them into a "none" bin. The motivation behind this was to improve integration of Porechop into our Galaxy workflows, where we typically don't want to run downstream steps on unbinned data.

It may be that there is a more robust way to do this than relying on the "none" assignment to read.barcode_call. I admit I didn't dig into the code deep enough to find out, as I wanted to roll this out quickly as part of a demonstration workflow.

[rrwick]

Thanks - it's merged!

I thought that there were now enough new bits to justify a new version, so this change (and a few others) are included in the just-released v0.2.3.