Implement arguments of some form for cell magics

[axch]

so it becomes possible to write something like

%%venturescript --debug-show-python-stack-trace
......

I don't know what the right interface is for this, exactly, or whether IPython already has facilities for making this sort of thing easy (without having to parse the cell for things that look like command line arguments).

The reason this would be useful is that a large fraction of the times that someone has wanted me to help debug a Venture crash in an iVenture notebook, I have wanted the Python stack trace in order to localize the error.

[fsaad]

I have a similar issue (now resolved) for dot command; it works but the parser for handling -- was a major hack which should probably rewritten.

#9