new option to invert switch state
Hey :)
I would like to thank you for your work !

I am using shelly 2.5 devices in switch mode to control and measure the power used by electric radiators/heaters.
By controlling a radiator/heater i mean switching it between PROGRAMMED (ECO) and DEFAULT (CONFORT) states, using its PILOT wire. This wire is quite common.

![image](https://user-images.githubusercontent.com/190756/85506905-96cd3c80-b5f1-11ea-8361-9e85d952a2bf.png)

When the first switch of the Shelly 2.5 has its relay OPENED, the pilot wire has no signal. Thus the radiator/heater is in ECO mode.
When the relay is closed, the pilot wire gets a normal 220V signal. Thus the radiator/heater is in CONFORT mode.

This means that to "switch off" (ie: in eco mode) the radiator/heater, the shelly switch must be ON (relay closed). This feels strange on UI ! Alterco does not provide this option on the switch itself. It would be nice that this plugin provides it !

It could be a config option on `device`:

```
 "devices": [
                {
                    "id": "BB3CC7",
                    "name": "xxx",
                    "switch1inverted": true, //Shelly 2.5 first switch
                    "switch2inverted": false, //Shelly 2.5 second switch
                }
            ]
```
