Automatic shell completion generation

* Add automatic shell completion generation.
Add automatic generation of shell completion scripts for various shells
(the ones supported by `clap_complete`). The scripts can be generated
using the `generate-shell-completion` xtask, which outputs the shell
script to stdout.

* Improve shell completion generation xtask.
General improvements to both the shell completion generation as well as
the xtask package itself. Update the README to match the new additions.

src/lib.rs

@@ -1,6 +1,7 @@
 use librespot_playback::audio_backend;
 
 pub const AUTHOR: &str = "Henrik Friedrichsen <henrik@affekt.org> and contributors";
+pub const BIN_NAME: &str = "ncspot";
 
 /// Return the [Command](clap::Command) that models the program's command line arguments. The
 /// command can be used to parse the actual arguments passed to the program, or to automatically