Capture std::clog writes and combine them with std::cerr writes (#989)

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

include/internal/catch_stream.h

@@ -21,6 +21,7 @@ namespace Catch {
 
     std::ostream& cout();
     std::ostream& cerr();
+    std::ostream& clog();
 
 
     struct IStream {