Integrate modvis.py to main pyan executable

[Technologicat]

It's sometimes useful to get a 30000ft (9144m) view of a codebase, looking only at module dependencies, especially when there are many small modules.

So ~some time~ already quite a while ago, I wrote a small, separate tool modvis.py, and archived it here.

But it's not yet integrated with the rest of Pyan; indeed, it's not even installed with the package.

Add a modvis mode as a new command-line option to pyan.