Add -dune-optional-output mode for dune's internal use Fixes #461 This PR adds a new command line flag that tells the driver not to write to the output file if there is no rewriting to be done. It's not 100% accurate if there are non context free transformations registered as we do not compare the AST for this feature but simply keep track of generated code via a hook. If any non context free transformation is registered, we simply assume it will rewrite something and always output. Signed-off-by: Nathan Rebours <nathan.p.rebours@gmail.com>