feat: add custom module to allow execution of external script

include/modules/custom.hpp

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <json/json.h>
+#include <gtkmm.h>
+#include <fmt/format.h>
+#include <thread>
+#include "util/chrono.hpp"
+#include "IModule.hpp"
+
+namespace waybar::modules {
+
+  class Custom : public IModule {
+    public:
+      Custom(std::string name, Json::Value config);
+      auto update() -> void;
+      operator Gtk::Widget &();
+    private:
+      Gtk::Label _label;
+      waybar::util::SleeperThread _thread;
+      const std::string _name;
+      Json::Value _config;
+  };
+
+}