Make pyroma a runnable module

[RuRo]

This PR adds a suitable __main__.py file to pyroma. This allows one to run pyroma as a module

python -m pyroma ...

Although the python -m syntax may seem verbose and redundant, it is useful for scripting/automation purposes as it allows you to ensure, which python environment/version is being used.

[RuRo]

@regebro sorry to bother you, but ping. Do you accept pull requests?

[regebro]

Thanks!