demo: add securedrop-source-cli crate

[redshiftzero]

(on top of #269, will retarget main when that is merged)

Towards #115

This just adds a crate with a CLI for the source side of the demo, right now with generate and show only to exercise the passphrase stuff in #269 (but send will need to be added):

$ cargo run --bin source-cli
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s
     Running `target/debug/source-cli`
Demo SecureDrop source client

Usage: source-cli <COMMAND>

Commands:
  generate  Generate a new source identity and print its recovery passphrase
  show      Rederive a source identity from its passphrase and print its public keys
  help      Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print help