Add the persistent_by_default configuration option

fix #3293

src/config/config.cpp

@@ -23,6 +23,14 @@ std::string Config::get(const std::string& option, const std::string& def)
   return it->second;
 }
 
+bool Config::get_bool(const std::string& option, const bool def)
+{
+  auto res = Config::get(option, "");
+  if (res.empty())
+    return def;
+  return res == "true";
+}
+
 int Config::get_int(const std::string& option, const int& def)
 {
   std::string res = Config::get(option, "");

src/config/config.hpp

@@ -44,6 +44,7 @@ public:
    * the second argument as the default.
    */
   static int get_int(const std::string&, const int&);
+  static bool get_bool(const std::string&, const bool);
   /**
    * Set a value for the given option. And write all the config
    * in the file from which it was read if save is true.