Make matplotlib an optional dependency?

[fedarko]

Not super important, but could be nice -- since some users might just want to create these matrices without visualizing them.

That said, the tradeoff here is we'd need to put in a lot of work to check that matplotlib is available before trying to import the stuff from _viz.py, and we'd need to update the pip install explanations to say "oh yeah install it as pip install wotplot[viz] or something. This seems potentially not worth the marginal benefit of not installing matplotlib in certain cases (esp since most users will probably already have it installed anyway)...

Update: with the advent of #14, we now use matplotlib in our tests, so this is gonna be more and more difficult to set up :|