CSS regression tests

[genoma]

https://github.com/garris/BackstopJS
https://css-tricks.com/automating-css-regression-testing/

[genoma]

@giacomozinetti @MakhBeth Needs to be tested on Windows do we have a Win* machine available?

I have used a separated bash script that npm should be able to run on Windows too, but I would love to do some proper testing before closing the issue.