Import .chart files

[mtolly]

Things to do:

• [x] Basic syntax parsing; previously did a version at https://github.com/mtolly/rhythm
• [x] Translation to MIDI
• [x] Implement the GH3 HOPO algorithm (IIUC: differs from RB in a single note after a chord that has that note can still be an auto-HOPO; in RB it is required that the note not be in the chord). This is needed because of the single "force" note, which just flips between strum/HOPO instead of the RB method of explicit strum/HOPO selection
• [x] Translate taps, forces, open notes

Export to .chart probably will not be necessary due to CH's full .mid support.

[mtolly]

Related to this -- CH allows MP3 audio in addition to Vorbis, which we don't support yet. Seems to be rarer, but see e.g. CTH2 which comes with 61 of them.