Markdown.Sanitizer removes <a href= and <img src= tags referencing relative paths

```
The part
(https?:\/\/|\/)
in Markdown.Sanitizer var img_white only allows full URLs:
http://... or https://...
or in-site URLs relative to the root
/...

As a consequence, the markdown code
"

producing the html code

is removed by Markdown.Sanitizer, because the in-site URL is relative to the
current path.

The same applies to var a_white

Solution:
Add an extra ? as follows:
(https?:\/\/|\/?)

```

Original issue reported on code.google.com by `geet...@yahoo.co.uk` on 9 Oct 2014 at 12:44