Introduce conditional wchar_t (and std::wstring) support

The support is turned on by default but the user might need to be able
to turn it off which is now possible by defining CATCH_CONFIG_NO_WCHAR.

include/internal/catch_session.h

@@ -30,7 +30,7 @@ namespace Catch {
         void useConfigData( ConfigData const& configData );
 
         int run( int argc, char* argv[] );
-    #if defined(WIN32) && defined(UNICODE)
+    #if defined(CATCH_CONFIG_WCHAR) && defined(WIN32) && defined(UNICODE)
         int run( int argc, wchar_t* const argv[] );
     #endif
         int run();