Option to remove cell numbers in output py module

[jph00]

[dmose]

@jph00 Knowing that you might well already have something (and that my patch might not land), I decided to work on this issue anyway, just because it seemed like a good bug for a new nbdev developer.

I've put together a patch that updates write_cells and ModuleMaker to do this. If you've already got something, no worries; I'm happy to let my patch go.

If not: I'm assuming this should be a user-visible feature. If that's true, how would you prefer to see it surfaced? Something in settings.ini with a corresponding command-line switch?