Add recursive descent merging with private notebook filtering

That is, something like,

nbmerge --recursive --exclude-private > merged.ipynb

BookBook also uses <number>-<name>.ipynb semantics for sorting. Specifically, it's a glob over -.ipynb (latex.py:143), lexicographically sorted. My convention uses lexicographic sorting, also. However, instead of that glob, it's aggregates all file that ending with .ipynb which do not begin with _. This lets me name files for inclusion as ###_Title_of_Notebook.ipynb. I think this is convenient because:

My eyes don't see the underscores;
You can produce formal titles by name.replace('_', ' '), stripping (optional) number prefix;
There are no spaces to worry about when performing shell actions.

jbn / nbmerge

Add recursive descent merging with private notebook filtering #2