Removed use of dynamic_cast for MultipleReporters

(Thanks to #630, #636 and #648)

include/internal/catch_interfaces_reporter.h

@@ -219,6 +219,7 @@ namespace Catch
         bool aborting;
     };
 
+    class MultipleReporters;
 
     struct IStreamingReporter : IShared {
         virtual ~IStreamingReporter();
@@ -247,6 +248,8 @@ namespace Catch
         virtual void testRunEnded( TestRunStats const& testRunStats ) = 0;
 
         virtual void skipTest( TestCaseInfo const& testInfo ) = 0;
+        
+        virtual MultipleReporters* tryAsMulti() { return CATCH_NULL; }
     };