passing --version now displays the features available

[scornejob]

First and foremost, this is a really cool project :) Thanks!

Here's a PR that implements #1255

spotifyd seba_dev ✗ cargo build --release --locked --features pulseaudio_backend
    Finished `release` profile [optimized] target(s) in 0.51s

spotifyd seba_dev ❯ ./target/release/spotifyd --version
spotifyd 0.4.2 (features: alsa_backend, pulseaudio_backend, dbus_mpris)
spotifyd seba_dev ❯ ./target/release/spotifyd -V
spotifyd 0.4.2 (features: alsa_backend, pulseaudio_backend, dbus_mpris)