Add link-<type> class

The module already detects the link type (internal, external, mail or frontpage). I thought it would be great if it could also add a css class accordingly so designers can take advantage. For example, in Wikipedia, all external links get a small icon to alert readers this link is an outside resource.

Patch attached!

I know HEAD is pretty unstable now, but it's a trivial patch anyway which I managed to test successfully on both HEAD and latest stable release.