Designing "plugin" interface about latexify_py

[odashi]

I don't plan to fix the final interface of plugins until the v1 release and we can start with ad-hoc implementations.

For now, the most significant issue is how we define additional expression rules for Call. We can start with only defining the interface to provide additinal function rules.

[ZibingZhang]

I believe array, ndarray, zeros, identity cases in ExprCodegen.visit_Call should belong in latexify.plugins.numpy.array

[odashi]

Yes I think so. Maybe sum and prod are also the candidates (and we can include them as a default plugin)