🚧 feat: autocompletion installer 🚧

[Benehiko]

- What I did
Override the cobra completion command to add extra functionality such as automatically installing completions on the user machine. The behavior of the completion command should still generate cobra commands.

- How I did it
Added a completion cobra.Command onto the root command.

- How to verify it
Run docker completion --help

Some examples:

docker completion zsh --manual
mkdir -p .docker/completions && docker completion zsh > .docker/completions/_docker
cat <<EOT >> .zshrc
# The following lines have been added by Docker to enable Docker CLI completions.
fpath=(.docker/completions/_docker $fpath)
autoload -Uz compinit
compinit
EOT
# End of Docker Completions⏎ 

Running the auto installer

⋊> ~/G/d/c/docker on completion-installer ⨯ go run . completion install

Detected shell [fish]

The automatic installer will do the following:

	mkdir -p /home/benehiko/.config/fish/completions
	docker completion fish > /home/benehiko/.config/fish/completions/docker.fish

Are you sure you want to continue? [y/N]

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 39.64758% with 137 lines in your changes missing coverage. Please review.

Project coverage is 61.34%. Comparing base (7f15dfa) to head (a123a86).
Report is 280 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5048      +/-   ##
==========================================
+ Coverage   61.08%   61.34%   +0.25%     
==========================================
  Files         295      298       +3     
  Lines       20660    21040     +380     
==========================================
+ Hits        12621    12907     +286     
- Misses       7142     7204      +62     
- Partials      897      929      +32