allow explicit un-initilised variant ctor

```c++
util::variant<int, double, std::string> var(util::no_init());
var.valid(); // false
util::variant<int, double, std::string>(util::no_init()).get_type_index() == util::detail::invalid_value; // true
```

test/unit.cpp

@@ -276,6 +276,7 @@ TEST_CASE( "variant default constructor", "[variant][default constructor]" ) {
     // As a result first type in Types... must be default constructable
     // NOTE: index in reverse order -> index = N - 1
     REQUIRE((util::variant<int, double, std::string>().get_type_index() == 2));
+    REQUIRE((util::variant<int, double, std::string>(util::no_init()).get_type_index() == util::detail::invalid_value));
 }
 
 TEST_CASE( "variant visitation", "[visitor][unary visitor]" ) {

variant.hpp

@@ -487,6 +487,8 @@ class printer : public static_visitor<>
 
 } // namespace detail
 
+struct no_init {};
+
 template<typename... Types>
 class variant
 {
@@ -503,12 +505,16 @@ class variant
 
 public:
 
+
     VARIANT_INLINE variant()
         : type_index(sizeof...(Types) - 1)
     {
         new (&data) typename detail::select_type<0, Types...>::type();
     }
 
+    VARIANT_INLINE variant(no_init)
+        : type_index(detail::invalid_value) {}
+
     template <typename T, class = typename std::enable_if<
                          detail::is_valid_type<T, Types...>::value>::type>
     VARIANT_INLINE explicit variant(T const& val) noexcept