Add sections.
Sections are an arbitrarily-long array of musical elements.
Sections may contain other sections.  This makes it really easy to compose elements
of a song together and really simplifies writing a long song.
Add example.
Finished transcribing the Song of Healing (possibly misidentified in previous commits
as the song of storms.  This is a good example of what my internal DSL/ internal
representation will look like in the final project.  I hope to take advantage of this
syntactic sugar when I start parsing.