cli: rename 'list' command to 'readers'; improve usage

There are going to be several `list` commands (e.g. `yubikey keys list`)
so this is a confusing name.

If we need more than one `readers` subcommand we can change this to be
`readers list` eventually.

Separately (in what probably should've been its own commit, mea culpa)
this adds slightly better usage.

cli/src/commands/readers.rs

@@ -0,0 +1,40 @@
+//! List detected readers
+
+use gumdrop::Options;
+use std::process::exit;
+use yubikey_piv::readers::Readers;
+
+/// The `readers` subcommand
+#[derive(Debug, Options)]
+pub struct ReadersCmd {}
+
+impl ReadersCmd {
+    /// Run the `readers` 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);
+        }
+    }
+}