cli: Initial yubikey-cli utility with list command

Adds a `yubikey-cli` crate to the workspace, with a `yubikey` binary,
which presently provides a `list` command for listing detected readers.

Dependencies:

- `env_logger`: logging
- `gumdrop`: argument parsing
- `termcolor`: colored terminal output

As this repo now contains a binary, it also checks in `Cargo.lock`.

Cargo.toml

@@ -13,7 +13,10 @@ license    = "BSD-2-Clause"
 repository = "https://github.com/iqlusioninc/yubikey-piv.rs"
 readme     = "README.md"
 categories = ["api-bindings", "cryptography", "hardware-support"]
-keywords   = ["ccid", "ecdsa", "rsa", "piv", "yubikey"]
+keywords   = ["ecdsa", "rsa", "piv", "pcsc", "yubikey"]
+
+[workspace]
+members = [".", "cli"]
 
 [badges]
 maintenance = { status = "experimental" }