v7.0.0

This release fixes some issues related to Song#setKey, which broke when the current song key was unknown.

• renames Song#setKey to #changeKey to clarify it not only changes the key in metadata, but also transposes all chords

• Song#setKey now only changes the current song key:

  • it changes the value in metadata
  • it updates any key directive in the song, or adds one if it was not present
  • it removes any key directive if the new value for key is null

• Song#changeKey will raise a clear error when the current song key is unknown