This also introduces Catch::clog() method to allow embedded targets
to override std::clog usage with their own stream (presumably null-sink),
similarly to how Catch::cout() and Catch::cerr() are used.
Fixes#989
- simpler, polymorphic hierarchy-based, approach
- less bitty conditionals spread across the code
- all resolved up-front so now config class is immutable
(it had evolved the way it was and in need of a clean-up sweep for a long time)
