Setting for device pixel ratio

Hi!

I didn't found information in readme about device pixel ratio. In `viewports` config we can set up `width` and `height`. But how about dpi?

I see that chrome start with `--force-device-scale-factor=1`, maybe we can start with another value?

Device pixel ratio needed for test sites, that uses `picture` tag or `srcset` attribute in `img`. For dpi=1, one image is used, and for dpi=2, other images are used. So, tests with only dpi=1 may not be entirely correct.