Allow users to enter private key (without showing it in CLI) per project, to re-use onion domains.

Suppose the onion domain is used quite extensively in some other software, or if it takes long to embed the onion domain in some server and/or client, a user may simply want to re-use an existing/previously used onion domain, without having to update all of its clients.

- [ ] Allow users to include a private key that is used to generate the `.onion` domain for specific project(s).