List all the available generators.

def generators():
    """
    List all the available generators.
    """
    from populous import generators

    base = generators.Generator

    for name in dir(generators):
        generator = getattr(generators, name)

        if isinstance(generator, type) and issubclass(generator, base):
            name = generator.__name__
            doc = (generator.__doc__ or '').strip()

            if doc:
                click.echo("{} - {}".format(name, doc))
            else:
                click.echo(name)