Introduce %M, %N template variables

- %M: total number of discs in the chosen release
- %N: number of current disc

Signed-off-by: JoeLametta <JoeLametta@users.noreply.github.com>

whipper/command/cd.py

@@ -60,6 +60,8 @@ disc and track template are:
  - %d: release title (with disambiguation)
  - %D: disc title (without disambiguation)
  - %I: MusicBrainz Disc ID
+ - %M: total number of discs in the chosen release
+ - %N: number of current disc
  - %T: medium title
  - %y: release year
  - %r: release type, lowercase