Make the equal pixels transparent

## Need

```
As a user
I need to see the differences immediately
So that I don't struggle to see the little ones
```
## Deliverables
- [ ] Possibility to make the equal pixels of the diff image transparent
- [ ] tests



## Solution

Adding another field to the `options object`:

``` js
options = {
transparency: {Number} between 0 - 255 // default 255
};
```

Modifying the [setPixel](https://github.com/gemini-testing/looks-same/blob/9ecfb9c9458283b1b259052c2d52a9f906301594/png.js#L80) method from the `prototype` of `PNGOut`, to receive another parameter, that will be set on the `alpha channel`, i.e `this._png.data[idx +3]`.