Make IStream::stream non-const

This way it makes much more sense from logically-const point
of view, and also means that concrete implementations don't
have to always have a `mutable` keyword on the stream member.

src/catch2/catch_config.cpp

@@ -108,8 +108,8 @@ namespace Catch {
         return m_data.reporterSpecifications;
     }
 
-    IStream const* Config::getReporterOutputStream(std::size_t reporterIdx) const {
-        return m_reporterStreams.at(reporterIdx).get();
+    IStream* Config::getReporterOutputStream(std::size_t reporterIdx) const {
+        return const_cast<IStream*>(m_reporterStreams.at(reporterIdx).get());
     }
 
     TestSpec const& Config::testSpec() const { return m_testSpec; }