Add the ability to control junit's showoutput attribute

There are situations in which the full output of the junit task is needed.
This happens typically when the test fails due to an internal error (as opposed to a test failure). For example, if you have a test case which doesn't have all required classes in its classpath, invoking this test will create a ClassNotFoundException. Other examples might include test cases which rely on the presence or execution of native code.
The stack trace for junit errors that occur before the test method is executed will only be visible if the showoutput attribute of the junit task is set to true.

I would suggest to either make the showoutput attribute configurable through an additional maven property and/or to set showoutput=true unconditionally if maven is executed in debug mode (-X option).