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/src/lib.rs

@@ -0,0 +1,14 @@
+//! `yubikey` command-line utility
+
+#![forbid(unsafe_code)]
+#![warn(
+    missing_docs,
+    rust_2018_idioms,
+    unused_lifetimes,
+    unused_qualifications
+)]
+
+#[macro_use]
+pub mod status;
+
+pub mod commands;