Plugin: Support for arbitrary (external) targets

Consider the following use case:

• There are bunch of radioactive content in our precious Drupal site
• There's another, non-Drupal server in the site complex, which has content we'd like to make radioactive
• We have a way to invoke radioactivity_add_energy from the non-Drupal server

So, what we need is a target that supports radioactivity for arbitrary string identifiers, e.g., url in the non-Drupal server.

The immediate advantages for this would be the following:

• Single implementation of radioactivity, i.e., no need to port it to the non-Drupal server
• Single point of configuration of radioactivity
• Comparable energy values between our Drupal content and the non-Drupal content

However, as the non-Drupal content are not nodes in the Drupal site, views-support for this feature might be a bit too much to ask.