Add support for shell completion

[Freed-Wu]

Fixes #608.

If you install keyring by pip install 'keyring[completion]', you can
generate shell completions by:

keyring --print-completion bash | sudo tee /usr/share/bash-completion/compleitons/keyring
keyring --print-completion zsh | sudo tee /usr/share/zsh/site-functions/_keyring
keyring --print-completion tcsh | sudo tee /etc/profile.d/keyring.csh

Note: the path of /usr/share is only for GNU/Linux mostly. If you use other OSs,
try:

• macOS, GUN/Linux (Homebrew): /usr/local/share
• Android (Termux): /data/data/com.termux/files/usr/share
• Windows (mingw64 of msys2): /mingw64/share
• ...

If you install keyring by your package manager (apt, pacman, nix, homebrew, etc),
these shell completions should have been packaged and you don't need generate
them by yourself.

After install shell completions, you can enable shell completions by:

• bash: install bash-completion, and
  . /usr/share/bash-completion/bash_completion in your bashrc.

• zsh: autoload -Uz compinit && compinit in your zshrc, and
  grep -w keyring ~/.zcompdump to check if it is installed correctly:

  $ grep -w keyring ~/.zcompdump
  'keyring' '_keyring'

[jaraco]

Releasing as 23.13.