Fixes ReDOS vulnerabilities.

Fixes ReDOS vulnerabilities.

Jamie Davis (@davisjam) from Virginia Tech reported that clean-css
suffers from ReDOS vulnerability [0] when fed with crafted input.

Since not so many people use clean-css allowing untrusted input such
cases may be rare, but this commit reworks vulnerable code to prevent
such attacks.

It also limits certain whitespace blocks to sane length of 31 characters
in validation regexes to prevent similar issues.

[0] https://snyk.io/blog/redos-and-catastrophic-backtracking