Plugins can be specified in three different ways:
* For a standard FindBugd distro, they can be put into the plugins directory
* For a JAWS distro, the file pluginlist.properties contains
  a list of URLs to plugins. These URLs can be relative or absolute. If they
  are absolute, they are relative to jar file that contained the pluginlist.properties
  file.
* You can define properties findbugs.plugin.*. Each such property defines a URL
  for a plugin