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

src/control.cc

@@ -93,7 +93,7 @@ Control::initialize() {
 void
 Control::cleanup() {
   //  delete m_scgi; m_scgi = NULL;
-  rpc::xmlrpc.cleanup();
+  rpc::rpc.cleanup();
 
   priority_queue_erase(&taskScheduler, &m_taskShutdown);