Never touch multipart/encrypted and multipart/signed (but allow -I).


Convert dates to the current locale and timezone (with an option?)


Allow -d and -p accept shell-like patterns and/or regular expressions:
-d content-*,-content-*-encoding


Add option --save-as='%format' to generate names for --save-* files. Develop
a DSL to conditionally use values of headers, parameters and the counter.