remember volume and track resume position

The app works really well, but I listen to a lot of trance mixes that can be up to 2 hours long. Being able to resume playback is essential to my MixCloud habit :)

It didn't take long to track down that MixCloud stores user preferences in local storage, or that WebView requires some private API tricks to retain local storage data.

Long term I'd like the app to remember the URL that was open rather than going to the MixCloud front page, but I'll submit that as a separate pull request if/when I get time.