Add modal chord progressions in their own directory

[ldrolez]

Problem: There are two ways to describe chord progressions. The classical one based on minor and major keys, and the modal notation which always refer to the relative major key. https://www.reddit.com/r/musictheory/comments/17hkgxw/wrong_chords_names/

Solution: Have 3 directories for chord progressions

• minor and major: which will use the classical notation
• modal: which will use the 'new' modal notation, relative to the major one

This will open great new possibilities and create less confusion!