Slightly improved link handling

This aligns radd.it's link handling with the style of the other expando types.
It also handles situations where the link is invalid (i.e. just `http://radd.it`).
The regex assumes that the only valid options are `...radd.it/r/...` and `...radd.it/magic/...` so if there are more I'll need to add them.

Currently, radd.it does not have a valid SSL cert, so these expandos will only work wihout HTTPS.