Issue when WP_PLUGIN_URL set

Hi,

I’m really liking this plugin, though I’ve found a tiny issue on the settings page when WP_PLUGIN_URL is defined as root relative. For instance I have in wp-config

define( 'WP_PLUGIN_URL', '/addons' );

So in the admin where file_get_contents is called it is looking in the file system not at an absolute web URL.

Changing line 77 to:
$prism_js_file_contents = file_get_contents( plugin_dir_path( __FILE__ ) . 'js/prism.js' );

Should resolve it and still work for more traditional installs.

Thanks
Phil