"External" URL support improvements

Problem/Motivation

Link Magician essentially ignores URLs that are "external" to the site.
The "URL Override" feature could be extended to allow for "external" urls (this seems fairly useful and shouldn't require a huge amount of effort)
It might also be rather useful to have an option to perform 'validation' on external URLs (which should be approached w/ caution in mind ... folks may not realize how many links they have on a site that are external, so at a minimum calling out that there may be considerations to bring up with your hosting service)

Proposed resolution

Figure out what should (or can) be added to the base Link Magician functionality as well consider how this plays in to adding Hooks/Events/Plugins.

Remaining tasks

1. Update the URL Override feature to support external urls: "External" URL support improvements

User interface changes

TBD.

API changes

TBD.