add ability to change menu_ prepend

This will allow 

```
@include('layouts.site-menu', ['items' => $siteMenu->roots()])
```

Instead of

```
@include('layouts.site-menu', ['items' => $menu_siteMenu->roots()])
```

Author: ChaosPower

config/menu.php

@@ -0,0 +1,5 @@
+<?php
+
+return [
+    'prepend' => 'menu_'
+];

src/Menu.php

@@ -66,7 +66,7 @@ public function make($name, $callback)
 
 			$this->collection->put($name, $menu);
 
-			$this->view->share('menu_'.$name, $menu);
+			$this->view->share(config('menu.prepend','menu_').$name, $menu);
 
 			return $menu;
 		}

src/MenusServiceProvider.php

@@ -5,6 +5,10 @@
 
 class MenusServiceProvider extends ServiceProvider {
 
+    public function boot()
+    {
+        $this->publishConfig();
+    }
 	/**
 	 * Indicates if loading of the provider is deferred.
 	 *
@@ -47,4 +51,11 @@ protected function registerServices()
 			return new Menu($app['config'], $app['view'], $app['html'], $app['url']);
 		});
 	}
+
+    protected function publishConfig()
+    {
+        $this->publishes([
+            __DIR__ . '/../config/menu.php' => config_path('menu.php'),
+        ], 'caffeinated-menu');
+    }
 }