Enable %B (barcode) and %C (catalog number) template variables

Already included but were not allowed.
Improved documentation.

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

whipper/command/cd.py

@@ -54,6 +54,8 @@ filling in the variables and adding the file extension. Variables for both
 disc and track template are:
  - %A: release artist
  - %S: release sort name
+ - %B: release barcode
+ - %C: release catalog number
  - %d: disc title
  - %y: release year
  - %r: release type, lowercase