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`.

cli/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "yubikey-cli"
+version = "0.0.0"
+description = """
+Command-line interface for performing encryption and signing using RSA and/or
+ECC keys stored on YubiKey devices.
+"""
+authors = ["Tony Arcieri <bascule@gmail.com>"]
+edition = "2018"
+license = "BSD-2-Clause"
+repository = "https://github.com/iqlusioninc/yubikey-piv.rs"
+readme     = "README.md"
+categories = ["command-line-utilities", "cryptography", "hardware-support"]
+keywords   = ["ecdsa", "rsa", "piv", "pcsc", "yubikey"]
+
+[dependencies]
+gumdrop = "0.7"
+env_logger = "0.7"
+lazy_static = "1"
+termcolor = "1"
+yubikey-piv = { version = "0.0.2", path = ".." }