Feature request: In debug mode, print full output of javac (including stdout)

Certain javac annotation processors and plugins like The Checker Framework sometimes print useful diagnostics on stdout.  Currently, there is no way to see this output using the Maven Compiler Plugin, even when Maven is run with the -X option.  I suggest that when Maven is in debug mode, the plugin should show as a debug message the full stdout of javac.  This should also work if javac is forked.

If someone points me at the relevant code, I am happy to try to submit a pull request for this change.