gsl::at overload for initializer_list

initializer_list do not have subscript operator, so the generic container overload of gsl::at fails to compile.
This commits adds an overload of gsl::at for initializer_lists, using *(initializer_list::begin()+index) instead of subscript operator

tests/at_tests.cpp

@@ -17,6 +17,7 @@
 #include <UnitTest++/UnitTest++.h> 
 #include <gsl.h>
 #include <vector>
+#include <initializer_list>
 
 using namespace std;
 using namespace gsl;
@@ -52,6 +53,16 @@ SUITE(at_tests)
 
         CHECK_THROW(at(a, 4), fail_fast);
     }
+
+    TEST(InitializerList)
+    {
+        std::initializer_list<int> a = { 1, 2, 3, 4 };
+
+        for (int i = 0; i < 4; ++i)
+            CHECK(at(a, i) == i+1);
+
+        CHECK_THROW(at(a, 4), fail_fast);
+    }
 }
 
 int main(int, const char *[])