Scraper to pull common chord progressions from HookTheory API.
All chord progressions are stored in output/processed.txt. Note that the starts of each chord progression are also stored there. For example, in a I-V-vi-IV chord progression, we would have separate entries for I, I-V, I-V-vi, and I-V-vi-IV. We need to store each step in the chord progression because we also store the probability of which chord comes next.