Offer individual character control for allow-unescaped-characters-in-url

Tomcat and JBossWeb offers specific invalid characters to be enabled via their ''tomcat.util.http.parser.HttpParser.requestTargetAllow' property. This can only be allowed as an all or nothing option with 'allow-unescaped-characters-in-url' in Undertow currently. An option for allowing individual characters like in Tomcat/JBossWeb would be preferred.