Add support for the other built-in code generators

[smeder]

Currently I don't see a way to get your plugin to create sources for C++ and Python, the other two built-in languages supported by protoc (--cpp_out, --python_out). Would be great to add support for those two options (unless I am missing something and one can force it via the plugin specification).

[smeder]

Figured out a (the?) way to do it:

protobufCodeGenPlugins = ["cpp:${protocPath}"]

seems to work. It would be nice though to get different output directories for each language...