Hold reporter's IConfig instance in the interface

Previously, every base derived from the IStreamingReporter had
its own `IConfig const* m_config` member, so this just centralizes
the handling thereof.

Part of #2061

src/catch2/catch_session.cpp

@@ -48,7 +48,7 @@ namespace Catch {
             // doesn't compile without a std::move call. However, this causes
             // a warning on newer platforms. Thus, we have to work around
             // it a bit and downcast the pointer manually.
-            auto ret = Detail::unique_ptr<IStreamingReporter>(new ListeningReporter);
+            auto ret = Detail::unique_ptr<IStreamingReporter>(new ListeningReporter(config));
             auto& multi = static_cast<ListeningReporter&>(*ret);
             auto const& listeners = Catch::getRegistryHub().getReporterRegistry().getListeners();
             for (auto const& listener : listeners) {