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/common/program.py

@@ -182,6 +182,8 @@ class Program:
         * ``%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
@@ -219,6 +221,8 @@ class Program:
             v['B'] = metadata.barcode
             v['C'] = metadata.catalogNumber
             v['c'] = metadata.releaseDisambCmt
+            v['M'] = metadata.discTotal
+            v['N'] = metadata.discNumber
             v['T'] = metadata.mediumTitle
             if metadata.releaseType:
                 v['R'] = metadata.releaseType