This applies also to the FindBugs, PMD etc. plugin.

I can configure settings for each of those plugins, but it's not possible to
define an overall configuration. For instance, I would like the build to be
unstable if the sum of all warnings from FindBugs, PMD and Checkstyle is greater
than 10.

This would of course only be valid for the installed plugins. I think the plugin
should be configured to one of these possibilities:

• Look only for itself, like it works now (that is: report the numbers to itself
  and decide for itself)
• Report the results to a global plugin which decides whether the project is
  stable/unstable etc.