Add -dune-optional-output mode for dune's internal use

[NathanReb]

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.