phpcs: Flag all uses of regex functions

[JDGrimes]

Because these can lead to security issues. See for example: https://blog.sucuri.net/2016/05/security-advisory-stored-xss-jetpack-2.html

[JDGrimes]

Actually, probably mostly need to worry about preg_replace_callback(), etc. But I guess all of them could cause issues.

[JDGrimes]

Like #134, this is another place where we need to be able to whitelist at times. So we may have to create a special sniff, or get that feature added upstream. It would be nice if whitelisting could be enabled per group.