Option to exclude some sub modules

[cgl]

It would be great to be able to exclude a given list of sub modules from the dependency graph.
This would enable us (1) to find the problematic modules, (2) get a better/simpler picture removing not very critical nodes from the graph.

[seddonym]

Agreed. Perhaps the simplest way to do this would be to support passing in a regular expression, something like:

impulse drawgraph mypackage --exclude=^mypackage\.foo*

Happy to consider a pull request for this! Or if you don't feel like doing that, it would be pretty easy to write a custom script based on Impulse's draw_graph function, which isn't very complicated.