feat: Define option URI.AllowedSymbols (#447)

library/HTMLPurifier/ConfigSchema/schema/URI.AllowedSymbols.txt

@@ -0,0 +1,7 @@
+URI.AllowedSymbols
+TYPE: string/null
+DEFAULT: '!$&\'()*+,;='
+--DESCRIPTION--
+If a system permits templated URLs, then the URI encoder may need extra
+hints about which symbols to preserve.
+--# vim: et sw=4 sts=4

library/HTMLPurifier/URI.php

@@ -108,7 +108,7 @@ class HTMLPurifier_URI
     public function validate($config, $context)
     {
         // ABNF definitions from RFC 3986
-        $chars_sub_delims = '!$&\'()*+,;=';
+        $chars_sub_delims = $config->get('URI.AllowedSymbols');
         $chars_gen_delims = ':/?#[]@';
         $chars_pchar = $chars_sub_delims . ':@';