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.rs

@@ -0,0 +1,98 @@
+//! Commands of the CLI application
+
+pub mod list;
+
+use self::list::ListCmd;
+use crate::status;
+use gumdrop::Options;
+use std::env;
+use std::process::exit;
+use termcolor::ColorChoice;
+
+/// The `yubikey` CLI utility
+#[derive(Debug, Options)]
+pub struct YubikeyCli {
+    /// Obtain help about the current command
+    #[options(short = "h", help = "print help message")]
+    pub help: bool,
+
+    /// Subcommand to execute.
+    #[options(command)]
+    pub command: Option<Commands>,
+}
+
+impl YubikeyCli {
+    /// Run the underlying command type or print usage info and exit
+    pub fn run(&self) {
+        // TODO(tarcieri): make this more configurable
+        status::set_color_choice(ColorChoice::Auto);
+
+        // Only show logs if `RUST_LOG` is set
+        if env::var("RUST_LOG").is_ok() {
+            env_logger::builder().format_timestamp(None).init();
+        }
+
+        match &self.command {
+            Some(cmd) => cmd.run(),
+            None => println!("{}", Commands::usage()),
+        }
+    }
+}
+
+/// Subcommands of this application
+#[derive(Debug, Options)]
+pub enum Commands {
+    /// `help` subcommand
+    #[options(help = "show help for a command")]
+    Help(HelpOpts),
+
+    /// `version` subcommand
+    #[options(help = "display version information")]
+    Version(VersionOpts),
+
+    /// `list` subcommand
+    #[options(help = "list detected readers")]
+    List(ListCmd),
+}
+
+impl Commands {
+    /// Run the given command
+    pub fn run(&self) {
+        match self {
+            Commands::Help(help) => help.run(),
+            Commands::Version(version) => version.run(),
+            Commands::List(list) => list.run(),
+        }
+    }
+}
+
+/// Help options
+#[derive(Debug, Options)]
+pub struct HelpOpts {
+    #[options(free, help = "subcommand to get help for")]
+    free: Vec<String>,
+}
+
+impl HelpOpts {
+    fn run(&self) {
+        if let Some(command) = self.free.first() {
+            if let Some(usage) = Commands::command_usage(command) {
+                println!("{}", usage);
+                exit(1);
+            }
+        }
+
+        println!("{}", Commands::usage());
+    }
+}
+
+/// Version options
+#[derive(Debug, Options)]
+pub struct VersionOpts {}
+
+impl VersionOpts {
+    /// Display version information
+    pub fn run(&self) {
+        println!("{} {}", env!("CARGO_PKG_NAME"), env!("CARGO_PKG_VERSION"));
+    }
+}