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")],
        },
    },
));
```

src/common/message_loop.rs

@@ -8,7 +8,9 @@ use cef::{Settings, api_hash, execute_process, initialize, shutdown, sys};
 ///
 /// - Windows and Linux: Support [`multi_threaded_message_loop`](https://cef-builds.spotifycdn.com/docs/106.1/structcef__settings__t.html#a518ac90db93ca5133a888faa876c08e0), so it is used.
 /// - macOS: Calls [`CefDoMessageLoopWork`](https://cef-builds.spotifycdn.com/docs/106.1/cef__app_8h.html#a830ae43dcdffcf4e719540204cefdb61) every frame.
-pub struct MessageLoopPlugin;
+pub struct MessageLoopPlugin {
+    pub config: CommandLineConfig,
+}
 
 impl Plugin for MessageLoopPlugin {
     fn build(&self, app: &mut App) {
@@ -19,7 +21,7 @@ impl Plugin for MessageLoopPlugin {
         let args = Args::new();
         let (tx, rx) = std::sync::mpsc::channel();
 
-        let mut cef_app = BrowserProcessAppBuilder::build(tx);
+        let mut cef_app = BrowserProcessAppBuilder::build(tx, self.config.clone());
         let ret = execute_process(
             Some(args.as_main_args()),
             Some(&mut cef_app),

src/lib.rs

@@ -16,21 +16,28 @@ use crate::mute::AudioMutePlugin;
 use crate::prelude::{IpcPlugin, NavigationPlugin, WebviewPlugin};
 use crate::zoom::ZoomPlugin;
 use bevy::prelude::*;
+use bevy_cef_core::prelude::CommandLineConfig;
 use bevy_remote::RemotePlugin;
 
 pub mod prelude {
     pub use crate::{CefPlugin, RunOnMainThread, common::*, navigation::*, webview::prelude::*};
+    pub use bevy_cef_core::prelude::CommandLineConfig;
 }
 
 pub struct RunOnMainThread;
 
-pub struct CefPlugin;
+#[derive(Debug, Default)]
+pub struct CefPlugin {
+    pub command_line_config: CommandLineConfig,
+}
 
 impl Plugin for CefPlugin {
     fn build(&self, app: &mut App) {
         app.add_plugins((
             LocalHostPlugin,
-            MessageLoopPlugin,
+            MessageLoopPlugin {
+                config: self.command_line_config.clone(),
+            },
             WebviewCoreComponentsPlugin,
             WebviewPlugin,
             IpcPlugin,