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.

Cargo.toml

@@ -7,7 +7,7 @@ application providing general-purpose public-key signing and encryption
 with hardware-backed private keys for RSA (2048/1024) and ECC (P-256/P-384)
 algorithms (e.g, PKCS#1v1.5, ECDSA)
 """
-authors    = ["Tony Arcieri <bascule@gmail.com>", "Yubico AB"]
+authors    = ["Tony Arcieri <tony@iqlusion.io>", "Yubico AB"]
 edition    = "2018"
 license    = "BSD-2-Clause"
 repository = "https://github.com/iqlusioninc/yubikey-piv.rs"