fix for #133

added value() function to get object value at given key or a default
value if key does not exist

doc/examples/basic_json__value.cpp

@@ -0,0 +1,29 @@
+#include <json.hpp>
+
+using namespace nlohmann;
+
+int main()
+{
+    // create a JSON object with different entry types
+    json j =
+    {
+        {"integer", 1},
+        {"floating", 42.23},
+        {"string", "hello world"},
+        {"boolean", true},
+        {"object", {{"key1", 1}, {"key2", 2}}},
+        {"array", {1, 2, 3}}
+    };
+
+    // access existing values
+    int v_integer = j.value("integer", 0);
+    double v_floating = j.value("floating", 47.11);
+
+    // access nonexisting values and rely on default value
+    std::string v_string = j.value("nonexisting", "oops");
+    bool v_boolean = j.value("nonexisting", false);
+
+    // output values
+    std::cout << std::boolalpha << v_integer << " " << v_floating
+              << " " << v_string << " " << v_boolean << "\n";
+}