Add JSON-RPC capability

Inline nlohmann/json for the JSON parsing itself, and handle requests
with the same SCGI interface as XML-RPC.

Based off the work in https://github.com/jesec/rtorrent

test/rpc/jsonrpc_test.h

@@ -0,0 +1,19 @@
+#include <cppunit/extensions/HelperMacros.h>
+
+#include "rpc/command_map.h"
+#include "rpc/jsonrpc.h"
+
+class JsonrpcTest : public CppUnit::TestFixture {
+  CPPUNIT_TEST_SUITE(JsonrpcTest);
+  CPPUNIT_TEST(test_basics);
+  CPPUNIT_TEST_SUITE_END();
+
+public:
+  void setUp();
+  void tearDown() {}
+
+  void test_basics();
+
+private:
+  rpc::JsonRpc m_jsonrpc;
+};