allow disabling of specific vendor assets

I spent way too long debugging why some FontAwesome icons weren't showing up, and it turned out that DebugBar's FontAwesome CSS was being loaded before mine. I ended up having to override both the DebugBar *and* JavascriptRenderer classes and override the protected property `$cssVendors` to get rid of DebugBar's FontAwesome CSS entirely. This patch makes it possible to more easily disable specific vendors.

```php
$debugBar->getJavascriptRenderer()->disableVendor('fontawesome');
$debugBar->getJavascriptRenderer()->disableVendor('highlightjs');
$debugBar->getJavascriptRenderer()->disableVendor('jquery');
```