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.
Implement command line parsing with GLib's parser. Two options
are available for now: -e to encrypt (and/or sign) and -d to
decrypt (and/or verify). Encrypt is the default mode and is the
only one supported for now.