add tojson subpackage for converting yaml docs to json (#171)

add tojson subpackage for converting yaml docs to json
merged-on-behalf-of: BBasile <BBasile@users.noreply.github.com>

examples/tojson/dub.json

@@ -0,0 +1,8 @@
+{
+    "name": "tojson",
+    "targetType": "executable",
+    "dependencies":
+    {
+        "dyaml": "*"
+    }
+}

examples/tojson/source/app.d

@@ -0,0 +1,54 @@
+module dyaml.tojson;
+import std.datetime;
+import std.json;
+import std.stdio;
+import dyaml;
+
+void main()
+{
+    auto doc = Loader.fromFile(stdin).load();
+    auto json = doc.toJSON;
+    writeln(json.toPrettyString);
+}
+
+JSONValue toJSON(Node node)
+{
+    JSONValue output;
+    if (node.isSequence)
+    {
+        output = JSONValue(string[].init);
+        foreach (Node seqNode; node)
+        {
+            output.array ~= seqNode.toJSON();
+        }
+    }
+    else if (node.isMapping)
+    {
+        output = JSONValue(string[string].init);
+        foreach (Node keyNode, Node valueNode; node)
+        {
+            output[keyNode.as!string] = valueNode.toJSON();
+        }
+    }
+    else if (node.isString)
+    {
+        output = node.as!string;
+    }
+    else if (node.isInt)
+    {
+        output = node.as!long;
+    }
+    else if (node.isFloat)
+    {
+        output = node.as!real;
+    }
+    else if (node.isBool)
+    {
+        output = node.as!bool;
+    }
+    else if (node.isTime)
+    {
+        output = node.as!SysTime.toISOExtString();
+    }
+    return output;
+}