fancyopts: add support for custom multi-arg opts in fancyopts.py This allows for more complex multi-arg opt logic, such as "--sum 1 --sum 2" -> 3, "--csv alice,bob --csv charlie" -> ["alice","bob","charlie"]. The current support for callables is insufficient for this. This is done by introducing a 'customopt' class which can be extended for more powerful opts logic. All existing opt-types are converted to use this class, simplifying the fancyopts() logic. Differential Revision: https://phab.mercurial-scm.org/D2090