Save proxies to a file.

async def save(proxies, filename):
    """Save proxies to a file."""
    with open(filename, 'w') as f:
        while True:
            proxy = await proxies.get()
            if proxy is None:
                break
            proto = 'https' if 'HTTPS' in proxy.types else 'http'
            row = '%s://%s:%d\n' % (proto, proxy.host, proxy.port)
            f.write(row)