Allow the convert plugin to convert to any format, not just mp3.

As requested in issue #362 I've changed the converter plugin to allow it to handle different formats and different conversion utilities.

Unfortunately it was necessary to change the configuration, the `ffmpeg` and `opts` properties have been removed, and instead replaced by `command` and `extension`. For example, here's what the default mp3 configuration looks like:

    convert:
        command: ffmpeg -i $source -y -aq 2 $dest
        extension: mp3

If you're happy with the technical aspects of this change, I'm happy to update the documentation.