Add install optional packages.

[rocky]

PyPI packaging show allow more installation options.

For example, if you don't have a Cs++ compiler, then pymathics-natlang can't get installed since it adds indirectly a dependency on murmurhash written in C++. So it might be nice to have be able to say:

pip install mathics-omnibus[no-c++]

There are lots of other combinations I can think of too. like "base" for just the base system.

https://setuptools.readthedocs.io/en/latest/userguide/dependency_management.html#optional-dependencies