Add two appsettings, DefaultPlayList and DoNotStream. DefaultPlayList al...

Allows the user to provide a playlist for the first time, useful if Spotbox has nothing remembered in usersettings from last time.  This may be better as a command line option?

DoNotStream is a half-hearted attempt at running SpotBox in a local test mode, where it doesn't actually stream but just manipulates the spotify session.  We should think to provide a test framework to obviate this, although I quite like the way Program.SelectAndPlayBestPlayList() is starting to behave.