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/commands/list.rs

@@ -0,0 +1,40 @@
+//! List detected readers
+
+use gumdrop::Options;
+use std::process::exit;
+use yubikey_piv::readers::Readers;
+
+/// The `list` subcommand
+#[derive(Debug, Options)]
+pub struct ListCmd {}
+
+impl ListCmd {
+    /// Run the `list` subcommand
+    pub fn run(&self) {
+        let mut readers = Readers::open().unwrap_or_else(|e| {
+            status_err!("couldn't open PC/SC context: {}", e);
+            exit(1);
+        });
+
+        let readers_iter = readers.iter().unwrap_or_else(|e| {
+            status_err!("couldn't enumerate PC/SC readers: {}", e);
+            exit(1);
+        });
+
+        if readers_iter.len() == 0 {
+            status_err!("no YubiKeys detected!");
+            exit(1);
+        }
+
+        for (i, reader) in readers_iter.enumerate() {
+            let name = reader.name();
+            let mut yubikey = match reader.open() {
+                Ok(yk) => yk,
+                Err(_) => continue,
+            };
+
+            let serial = yubikey.serial();
+            println!("{}: {} (serial: {})", i + 1, name, serial);
+        }
+    }
+}