Proposed fixes to NR A.2 Common Chord Modifiers

[Johan Vromans]

As of 2.19.28, chord modifier 5 is interpreted to mean a power chord. This
is not yet reflected in the NR.

I propose the following changes.

Type "Major", modifier "nothing" and example "c1" with lily code

  \chords { c1 }
  \chordmode { c1 }

Type "Minor", modifier "m".

These changes are mandatory since the current content conflicts with the
current implementation.

Type "Power chord (two-voiced)", modifier "5", example
"\powerChords c1:5" and lily code 

  \chords { c1:5 }
  \chordmode { \powerChords c1:5 }

Type "Power chord (three-voiced)", modifier "5.8", example
"\powerChords c1:5.8" and lily code 

  \chords { c1:5.8 }
  \chordmode { \powerChords c1:5.8 }

-- Johan