Pass extra options to prove on the command line.

When invoking the build process with -Dprove.args=-v we'll be able to
see all the log messages spit during our tests.
Pass -q as the default prove verbosity.

Maven misbehaves if the parameter is null, so we pass the default "-q".
Merge pull request #17 from piojo/verbose-tests

Pass extra options to prove on the command line.