There's an app called Free Download Manager which has this feature. I suspect it may be using youtube-dl under the hood, since the functionality looks similar (with a GUI, of course). The app allows you to copy/paste youtube urls and it parses the page for available downloads (audio and video). I use it regularly for putting technical talks onto my phone to watch or listen to on my commute.

The app is very easy to use, and one of the few I would recommend to a non-technical person for the features it has.

I know the name makes it sound like spyware, but it's the most lightweight tool I've found for both this and the occasional torrent.

http://www.freedownloadmanager.org/