feat: Add configurable CEF command line flags (#16)

## Summary
- Add CommandLineConfig struct for CEF command line switches
- Use direct struct initialization with optional helper methods
- Change default to secure: only use-mock-keychain enabled on macOS debug builds
- Add comprehensive documentation with usage examples

## Usage
```rust
use bevy_cef::prelude::*;

// Default (secure, includes use-mock-keychain on macOS debug)
app.add_plugins((DefaultPlugins, CefPlugin::default()));

// Add switches while preserving defaults (recommended)
app.add_plugins((
    DefaultPlugins,
    CefPlugin {
        command_line_config: CommandLineConfig::default()
            .with_switch("disable-gpu")
            .with_switch_value("remote-debugging-port", "9222"),
    },
));

// Full customization with direct initialization
app.add_plugins((
    DefaultPlugins,
    CefPlugin {
        command_line_config: CommandLineConfig {
            switches: vec!["disable-gpu"],
            switch_values: vec![("remote-debugging-port", "9222")],
        },
    },
));
```

examples/brp.rs

@@ -13,7 +13,7 @@ fn main() {
         .add_plugins((
             DefaultPlugins,
             RemotePlugin::default().with_method("greet", greet),
-            CefPlugin,
+            CefPlugin::default(),
         ))
         .add_systems(
             Startup,

examples/custom_material.rs

@@ -12,7 +12,7 @@ fn main() {
     App::new()
         .add_plugins((
             DefaultPlugins,
-            CefPlugin,
+            CefPlugin::default(),
             WebviewExtendMaterialPlugin::<CustomExtension>::default(),
         ))
         .add_systems(Startup, (spawn_camera, spawn_webview))

examples/devtool.rs

@@ -10,7 +10,7 @@ use bevy_cef::prelude::*;
 
 fn main() {
     App::new()
-        .add_plugins((DefaultPlugins, CefPlugin))
+        .add_plugins((DefaultPlugins, CefPlugin::default()))
         .add_systems(
             Startup,
             (spawn_camera, spawn_directional_light, spawn_webview),

examples/host_emit.rs

@@ -7,7 +7,7 @@ use std::time::Duration;
 
 fn main() {
     App::new()
-        .add_plugins((DefaultPlugins, CefPlugin))
+        .add_plugins((DefaultPlugins, CefPlugin::default()))
         .add_systems(
             Startup,
             (spawn_camera, spawn_directional_light, spawn_webview),

examples/js_emit.rs

@@ -8,7 +8,7 @@ fn main() {
     App::new()
         .add_plugins((
             DefaultPlugins,
-            CefPlugin,
+            CefPlugin::default(),
             JsEmitEventPlugin::<Message>::default(),
         ))
         .add_systems(

examples/navigation.rs

@@ -11,7 +11,7 @@ use bevy_cef::prelude::*;
 
 fn main() {
     App::new()
-        .add_plugins((DefaultPlugins, CefPlugin))
+        .add_plugins((DefaultPlugins, CefPlugin::default()))
         .add_systems(
             Startup,
             (spawn_camera, spawn_directional_light, spawn_webview),

examples/preload_scripts.rs

@@ -7,7 +7,7 @@ use bevy_cef::prelude::*;
 
 fn main() {
     App::new()
-        .add_plugins((DefaultPlugins, CefPlugin))
+        .add_plugins((DefaultPlugins, CefPlugin::default()))
         .add_systems(
             Startup,
             (spawn_camera, spawn_directional_light, spawn_webview),

examples/simple.rs

@@ -5,7 +5,7 @@ use bevy_cef::prelude::*;
 
 fn main() {
     App::new()
-        .add_plugins((DefaultPlugins, CefPlugin))
+        .add_plugins((DefaultPlugins, CefPlugin::default()))
         .add_systems(
             Startup,
             (spawn_camera, spawn_directional_light, spawn_webview),

examples/sprite.rs

@@ -5,7 +5,7 @@ use bevy_cef::prelude::*;
 
 fn main() {
     App::new()
-        .add_plugins((DefaultPlugins, CefPlugin))
+        .add_plugins((DefaultPlugins, CefPlugin::default()))
         .add_systems(Startup, (spawn_camera_2d, spawn_sprite_webview))
         .run();
 }

examples/zoom_level.rs

@@ -6,7 +6,7 @@ use bevy_cef::prelude::*;
 
 fn main() {
     App::new()
-        .add_plugins((DefaultPlugins, CefPlugin))
+        .add_plugins((DefaultPlugins, CefPlugin::default()))
         .add_systems(
             Startup,
             (spawn_camera, spawn_directional_light, spawn_webview),