sounds.go doesn't have a copyright header, because the file is generated
by a program. I don't want to make it output the copyright header,
because users can generate sounds themselves, and the copyright status
of such generated files can be different.

The new command replaces old test command-line utilities from cmd/image
and cmd/audio directories. It can now generate both image and audio
captchas, and accepts a few useful flags.

Edit .gitignore to reflect changes.

Next commits will introduce perturbations.