added option -p to plot results with matplotlib

You may plot with

```bash
python3 simple.py -p plot.png
```

To adjust image resolution for displaying on the web adjust `dpi` variable when saving the file

https://github.com/dudarev/aiozmq/compare/aio-libs:master...dudarev:master#diff-53fedcad495e9a0c5d4d49189c082363R313