Control capturing in rsync_project

[bitprophet]

contrib.project.rsync_project can't control whether its local call to rsync captures or doesn't capture, so right now it uses the default behavior -- not capturing.

However, when combined with hide('stdout', 'stderr') or any combo thereof, this means it's impossible to debug rsync problems, because noncaptured local + output hiding == pipe to stderr.

Make it possible to fix this by adding a passthrough capture kwarg to rsync_project.