Improve support for reserved characters

[acoburn]

Certain ASCII characters require special handling in the context of URLs, URIs and IRIs. These are outlined in RFC 3987 and include characters such as [ and ] or %. It should be possible to completely disallow certain such characters while URL-encoding those that are allowed. Since non-ASCII characters are allowed, it should be possible to limit the URL encoded versions to just these characters. These character lists should also be configurable.