Support optimization targets in OPB formulas

[marcvinyals]

Would you consider adding support for optimization functions in OPB formulas? This would essentially involve adding a member variable and a corresponding method "set_optimization_target". If the target has been set then the output will contain a "min:" line, otherwise there is no change in observable behaviour.

I am happy to implement this and send a PR if it is in scope.

This is related to #73, which can probably be closed.