FIX argparse bug

[jbn]

When I used a pattern I don't often use,

nbmerge f1.ipynb f2.ipynb > output.ipynb

the program failed. For some reason, the full path to the nbmerge script is being included in the array argparse aggregates for the files parameter. I quickly fixed it with a kludge (see: 03dcbec). But, I need to figure out why argparse is behaving this way, then fix it correctly.