Support encrypting a file to multiple recipients

As far as I can tell, "keybase encrypt" currently limits you to a single "them" value. I fairly often have email discussions with multiple people; being able to use the keybase client to encrypt to all of them would be nice.

The issue names the `keybase encrypt` command and its single `them` value, but no files or tests. Start by locating that command's argument handling and encryption path, then trace how recipients are represented. Done means the command can target multiple recipients while preserving the existing single-recipient behavior.

Written by the indexing model from the issue text.