python -m support

```console
/usr/local/pyenv/versions/3.8.18/bin/python3: No module named gitlint.__main__; 'gitlint' is a package and cannot be directly executed
```
It'd be handy to be able to invoke `gitlint` via `python -m gitlint`.
This can be supported by adding a `__main__.py` which calls `gitlint.cli:cli`.