Add float/int literal for Approx

include/internal/catch_approx.h

@@ -25,6 +25,8 @@ namespace Detail {
 
         static Approx custom();
 
+        Approx operator-() const;
+
         template <typename T, typename = typename std::enable_if<std::is_constructible<double, T>::value>::type>
         Approx operator()( T const& value ) {
             Approx approx( static_cast<double>(value) );
@@ -121,7 +123,12 @@ namespace Detail {
         double m_scale;
         double m_value;
     };
-}
+} // end namespace Detail
+    
+namespace literals {
+    Detail::Approx operator "" _a(long double val);
+    Detail::Approx operator "" _a(unsigned long long val);
+} // end namespace literals
 
 template<>
 struct StringMaker<Catch::Detail::Approx> {