Give the possibility to set a list of device

Hi everybody, 

My first Pull Request for your amazing plugin. I hope my contribution is good enough. Feedback is welcome. 

Instead of the ```<device>``` parameter, we can now specify a list of devices thanks to the ```<devices>``` parameter : 
```
<devices>
    <device>serial1</device>
    <device>serial2</device>
</devices>
```

Manu