Improve validation of CSS selector-based cosmetic filters

Reported internally. The following invalid filter was not
discarded by uBO:

    123tvnow.com##.123tv-ads

The correct form should be:

    123tvnow.com##.\31 23tv-ads

Not discarding invalid CSS selector-based cosmetic
filter may break CSS selector-based cosmetic
filtering.

src/js/static-ext-filtering.js

@@ -96,7 +96,7 @@
         }
         // Quick regex-based validation -- most cosmetic filters are of the
         // simple form and in such case a regex is much faster.
-        var reSimple = /^[#.][\w-]+$/;
+        var reSimple = /^[#.][A-Za-z_][\w-]*$/;
         return function(s) {
             if ( reSimple.test(s) ) { return true; }
             try {