Add purge-css webpack plugin

[ilyaryabchinski]

Is your feature request related to a problem? Please describe.
If I import big amount of css (e.g. frameworks like Bootstrap etc.), a lot of unused css classes end up in the css prod build.

Describe the solution you'd like
I think that purge-css can fix this.

Describe alternatives you've considered
Maybe some of already used css plugins (like cssnano) can also fix the problem.