Specify audiobooks base in config file

[pganssle]

Right now, using audio-feeder update you need to specify the path to the audiobooks directory.

The reasoning behind this was that eventually this software will support retrieving metadata for things like audio courses and other such things, and you'd need to specify whether the path you are updating is books or courses, and you'd only want to parse the directory containing that subset.

I think the better approach than "specify by command line" is to specify in either the config or the database (I'm leaning towards config) which directories contain audiobooks and whatnot (probably just a top level directory which can contain symlinks), and the update script can just update everything.