Expose CMake binaries

[diegoferigo]

Fixes #13

This PR introduces a new expose_binaries option that allows specifying a relative path to binaries built with CMake to expose to the Python environment. It is implemented with a generic __main__.py magic file that calls the C++ executable.

Projects that want to expose a binary just need to:

1. Use the new option as expose_binaries=["bin/<executable_name>"]
2. Create a new entrypoint:

   [options.entry_points]
   console_scripts =
       executable_name = package_name.bin.__main__:main

Check the SWIG example for a MWE.