add .natvis for MSVC debug view

CMakeLists.txt

@@ -35,7 +35,14 @@ target_include_directories(
     ${NLOHMANN_JSON_TARGET_NAME}
     INTERFACE $<INSTALL_INTERFACE:include/>
 )
-            
+
+target_sources(
+    ${NLOHMANN_JSON_TARGET_NAME} 
+    INTERFACE 
+        $<INSTALL_INTERFACE:include/json.natvis>
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/${NLOHMANN_JSON_SOURCE_DIR}/json.natvis>  
+)
+           
 ##
 ## TESTS
 ## create and configure the unit test target

src/json.natvis

+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+    <Type Name="nlohmann::basic_json&lt;*&gt;">
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::null">null</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::object">{*(m_value.object)}</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::array">{*(m_value.array)}</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::string">{*(m_value.string)}</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::boolean">{m_value.boolean}</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::number_integer">{m_value.number_integer}</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::number_unsigned">{m_value.number_unsigned}</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::number_float">{m_value.number_float}</DisplayString>
+        <DisplayString Condition="m_type == nlohmann::detail::value_t::discarded">discarded</DisplayString>
+        <Expand>
+            <ExpandedItem Condition="m_type == nlohmann::detail::value_t::object">
+                *(m_value.object),view(simple)
+            </ExpandedItem>
+            <ExpandedItem Condition="m_type == nlohmann::detail::value_t::array">
+                *(m_value.array),view(simple)
+            </ExpandedItem>
+        </Expand>
+    </Type>
+    
+    <!--    skip the pair first/second members in the treeview while traversing a map. 
+            Only works in VS 2015 Update 2 and beyond using the new visualization -->
+    <Type Name="std::pair&lt;*, nlohmann::basic_json&lt;*&gt;&gt;" IncludeView="MapHelper">
+        <DisplayString>{second}</DisplayString>
+        <Expand>
+            <ExpandedItem>second</ExpandedItem>
+        </Expand>
+    </Type>
+    
+</AutoVisualizer>