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/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))