Spawn a debug shell.

def shell(self):
        """
        Spawn a debug shell.
        """
        click.echo(click.style("NOTICE!", fg="yellow", bold=True) + " This is a " + click.style("local", fg="green", bold=True) + " shell, inside a " + click.style("Zappa", bold=True) + " object!")
        self.zappa.shell()
        return