Pipelines: ability to generate make target easily

[hartym]

To replace this snippet:

# Pipelines
@listen(make.on_generate)
def on_make_generate_pipelines(event):
    # Releases
    event.makefile.add_target(
        'release',
        '''
            python -c 'import medikit; print(medikit.__version__)' || pip install medikit;
            $(PYTHON) -m medikit pipeline release start
        ''',
        phony=True,
        doc='Releases medikit.'
    )