#3 Extend & document playlist format

+shuffle
Song1.mp3
Song2.mp3

+shuffle

+group
 +shuffle

 Song1.mp3
 Song2.mp3
 Song3.mp3
-group

+group
 Song4.mp3
 Song5.mp3
-group

+shuffle
$artist: Artist1

+shuffle-mode less-random
+shuffle

+pause 5ms

+volume 150%
Song1.mp3

+volume 2x
Song2.mp3

+fade-in
+fade-out
Song3.mp3

The basic idea behind the playlist format is that it consists of just a bunch of lines, which are either songs, or if they start with a `+`, `-`, or `$`, they have special meaning. So something like ``` +shuffle Song1.mp3 Song2.mp3 ``` Is a basic playlist that shuffles its constituent songs, and this is currently implemented. However, features that I want to implement include something like the following: ``` +shuffle +group +shuffle Song1.mp3 Song2.mp3 Song3.mp3 -group +group Song4.mp3 Song5.mp3 -group ``` which will shuffle between playing a shuffled group of `Song1`, `Song2`, and `Song3`, and a non-shuffled group of `Song4` and `Song5`. The `$` will be used like this: ``` +shuffle $artist: Artist1 ``` Which will create a shuffled playlist of all songs from `Artist1`. Finally, there will be additional `+` attributes, to apply to single songs, or potentially groups, such as ``` +shuffle-mode less-random +shuffle +pause 5ms +volume 150% Song1.mp3 +volume 2x Song2.mp3 +fade-in +fade-out Song3.mp3 ```