Add Difficulty Level about cozmo-song-match

[gbroques]

Another possible scheme:

Easy

• Increment by one, and only play 1/3 of the song.

Medium

• Play 2/3 of the song and increment by 2 after reaching 1/3 of the song

Hard

• Play the entire song, increment by 3 after reaching 2/3 of the song

The only thing I don't like about this is that the song might not be recognizable for easy and medium difficulties. One way to remedy this is by coding an easy, medium, and hard sequence for each song. This sounds a bit cumbersome though.