support multiple recipients for keybase encrypt

It should be possible to use `keybase encrypt` to encrypt something for multiple recipients, see eg.

http://laurent.bachelier.name/2013/03/gpg-encryption-to-multiple-recipients/

Currently, `keybase encrypt` just uses the first recipient on the "them" parameter and ignores the rest.

Start with the keybase encrypt command and inspect how its "them" parameter is processed; the issue reports that only the first recipient is used. Compare the intended behavior with the linked multi-recipient encryption example, and consider the work complete when one invocation encrypts successfully for every supplied recipient.

Written by the indexing model from the issue text.