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.hpp

@@ -103,6 +103,9 @@ namespace Catch {
     std::ostream& cerr() {
         return std::cerr;
     }
+    std::ostream& clog() {
+        return std::clog;
+    }
 #endif
 }