Improve usability of --format

[bjornua]

The program should be pretty user friendly yet flexible. The --target option is flexible but not very easy to read. The --target option should probably be more verbose, show hierarchy, and use named arguments

Inpsired by rust structs .transcoderc file could be in a format like:

mypreset = mkv {
    video: maxres {
        maxheight: 1024,
        maxwidth: 768,
        encoder: h264 {
            crf: 18,
            speed: normal
        }
    ),
    audio: opus {
        bitrate: 192
    }
}

If it's very hard to implement an parser, consider using some json format.