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

@@ -4,12 +4,12 @@ use gumdrop::Options;
 use std::process::exit;
 use yubikey_piv::readers::Readers;
 
-/// The `list` subcommand
+/// The `readers` subcommand
 #[derive(Debug, Options)]
-pub struct ListCmd {}
+pub struct ReadersCmd {}
 
-impl ListCmd {
-    /// Run the `list` subcommand
+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);