parse tag attributes that contain dashes

The regex is wrong, it will also match attr names like `a-` or `-b?` or
even `-`, which are of course not valid.

Since the whole concept of sacy is centered around parsing
developer-supplied html and doesn't use a full-blown parser, I feel
the compromise is acceptable here in order to keep the regex simple.

src/sacy/sacy.php

@@ -81,7 +81,9 @@ function workUnitFromTag($tag, $attrdata, $content){
     }
 
     private function extract_attrs($attstr){
-        $attextract = '#([a-z]+)\s*=\s*(["\'])\s*(.*?)\s*\2#';
+        // The attribute name regex is too relaxed, but let's
+        // compromise and keep it simple.
+        $attextract = '#([a-z\-]+)\s*=\s*(["\'])\s*(.*?)\s*\2#';
         if (!preg_match_all($attextract, $attstr, $m)) return false;
         $res = array();
         foreach($m[1] as $idx => $name){