nofollow filter for external links
Based on [svetlyak](http://www.djangosnippets.org/users/svetlyak/)'s [nofollow filter](http://www.djangosnippets.org/snippets/312/). This one processes only external URL's. Links with internal URL's will be returned unmodified. There's one gotcha; I preferred to have false positives instead of false negatives. So, it will nofollow `href="some/relative/path/"` for example. If you must do relative paths do it like this: <a href="./some/relative/path">link text</a>
- filter
- nofollow