make ciphernet a little more friendly to use

if input is not a tty, and there where no other options and no username provided,
treat the input as an encrypted message.

so encryption is `$input | cipherhub $username` and decryption is `$input | cipherhub`
passing `-d` still works, of course.

Also, I've added a friendlier error message.
working on npmd has taught me the importance of helpful error messages.

this pull request depends on my other pull request to rsa-stream https://github.com/substack/rsa-stream/pull/2