proposal for emplace/emplace_back (#349)

doc/examples/emplace.cpp

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+#include <json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json object = {{"one", 1}, {"two", 2}};
+    json null;
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << null << '\n';
+
+    // add values
+    object.emplace("three", 3);
+    null.emplace("A", "a");
+    null.emplace("B", "b");
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << null << '\n';
+}

doc/examples/emplace.link

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+<a target="_blank" href="http://melpon.org/wandbox/permlink/Qg5Ogrh8yFrwT2GY"><b>online</b></a>

doc/examples/emplace.output

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+{"one":1,"two":2}
+null
+{"one":1,"three":3,"two":2}
+{"A":"a","B":"b"}

doc/examples/emplace_back.cpp

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+#include <json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json array = {1, 2, 3, 4, 5};
+    json null;
+
+    // print values
+    std::cout << array << '\n';
+    std::cout << null << '\n';
+
+    // add values
+    array.emplace_back(6);
+    null.emplace_back("first");
+    null.emplace_back(3, "second");
+
+    // print values
+    std::cout << array << '\n';
+    std::cout << null << '\n';
+}

doc/examples/emplace_back.link

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+<a target="_blank" href="http://melpon.org/wandbox/permlink/jdch45YEMX94DvlH"><b>online</b></a>

doc/examples/emplace_back.output

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+[1,2,3,4,5]
+null
+[1,2,3,4,5,6]
+["first",["second","second","second"]]