refactor(script): Split non-/tail logic

Split the script module into one tailing output module and
one exec once module.

Refs #281

include/modules/cmdscript.hpp

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "modules/script.hpp"
+
+POLYBAR_NS
+
+namespace modules {
+  class cmdscript_module : virtual public script_module {
+   public:
+    explicit cmdscript_module(const bar_settings&, string);
+
+   protected:
+    void process();
+  };
+}
+
+POLYBAR_NS_END