feat(loading): robust error reporting and mutation

cfg now reports which fields are missing when loading. Further, the mutate method has been added for easier mutation of the underlying configuration struct

readme.md

@@ -33,10 +33,12 @@ and strongly typed.
 #include "fourdst/config/config.h"
 #include <string>
 #include <print>
+#include <optional>
 
 struct MyPhysicsOptions {
     int gravity = 10;
     float friction = 0.5f;
+    std::optional<float> dampening = 0.1f;
     bool enable_wind = false;
 };
 
@@ -64,12 +66,38 @@ int main() {
     // You can load a config from a file
     try {
         cfg.load("my_config.toml");
+        // You can also pass an optional bool as a second argument to turn on verbose error 
+        // reporting. This will display a tree of missing or invalid fields. Note that due to limitations
+        // in C++'s ability to detect default iniailized values vs initializer list values 
+        // missing fields which you set an initializer list for are still considered missing. 
+        // **ONLY fields marked with std::optional are exempt from this rule.**
     } catch (const fourdst::config::exceptions::ConfigError& e) {
         std::println("Error loading config: {}", e.what());
     }
     
     // You can access the config values
     std::println("My Simulation Name: {}, My Simulation Gravity: {}", cfg->name, cfg->physics.gravity);
+    
+    // libconfig intentioanlly discourages direct modification of config values. However, if you need 
+    // to modify values after loading them you can use the mutate function. This takes a lambda
+    // which recives a mutable reference to the underlying config struct.
+    
+    cfg.mutate([](MySimulationConfig& config) {
+        config->physics.enable_wind = true;
+    });
+    
+    // Making these mutations will put the config into a "MODIFIED" state and will cache the unmodified values. 
+    // You can reset the state and revert to the unmodified values with the reset function.
+    
+    cfg.reset();
+    
+    // The current state of the config can be checked with the get_state() function or the describe_state() function.
+    // get_state returns an enum value while describe_state returns a human readable string.
+    
+    std::println("Config State: {}", cfg.describe_state());
+    fourdst::config::ConfigState state = cfg.get_state();
+    
+    // Possible states are DEFAULT, LOADED_FROM_FILE, and MODIFIED
 }
 ```