Added generic singletons facility

<sigh> yes, I know - but we have them - may as well make them consistent and safer

include/internal/catch_singletons.cpp

@@ -0,0 +1,36 @@
+/*
+ *  Created by Phil Nash on 15/6/2018.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "catch_singletons.hpp"
+
+#include <vector>
+
+namespace Catch {
+
+    namespace {
+        static auto getSingletons() -> std::vector<ISingleton*>*& {
+            static std::vector<ISingleton*>* g_singletons = nullptr;
+            if( !g_singletons )
+                g_singletons = new std::vector<ISingleton*>();
+            return g_singletons;
+        }
+    }
+
+    ISingleton::~ISingleton() {}
+
+    void addSingleton(ISingleton* singleton ) {
+        getSingletons()->push_back( singleton );
+    }
+    void cleanupSingletons() {
+        auto& singletons = getSingletons();
+        for( auto singleton : *singletons )
+            delete singleton;
+        delete singletons;
+        singletons = nullptr;
+    }
+
+} // namespace Catch