Support python -m github_to_sqlite usage

[kurtmckee]

Not all environments can support installed scripts. For example, zip files:

python -m pip install . --target demo

cd demo
zip -r ../demo.zip .
cd ..

export PYTHONSAFEPATH=1
export PYTHONPATH=demo.zip

python -c 'import github_to_sqlite as gts; print(gts)'
# Output:
# module 'github_to_sqlite' from '/.../demo.zip/github_to_sqlite/__init__.py'>

python -m github_to_sqlite

Having a __main__.py file enables less common use cases.