Allow to verify detached signature, but only by assuming the
corresponding signed text is in a file with the same basename
than the signature itself, minus the '.sig', '.asc', or '.gpg'
extension. If such a file does not exist, abort.
Add a draft implementation of the -d, --decrypt command (which is
used both to decrypt and/or to verify). The core functionality is
there, but the UI could probably be better, both in terms of
aesthetics and in terms of informations shown about the decrypted
or verified file.