Describe the feature or problem you’d like to solve

When running the completion sub-command, gh cli will check for updates and write a message if an update is available (this was suggested in #85). The expected use is to eval this command in your .bashrc or .anything_rc, this means that a information message will be written every time a prompt is opened.

While I think this feature is useful (this tool is not part of most distributions official repositories yet), I also think that it can be considered to be quite annoying and intrusive. If you installed this from a repository, this also means that you might get pointless messages for a few days until your package gets updated, this might become a very common scenario in the future.

Proposed solution

It would be nice to have a feature switch in parameters (-q, --quiet for example?) to disable update notifications. Currently no such option seem to exist:

>> gh completion -h
Generate shell completion scripts for GitHub CLI commands.

For example, for bash you could add this to your '~/.bash_profile':

	eval "$(gh completion)"

When installing GitHub CLI through a package manager, however, it's possible that
no additional shell configuration is necessary to gain completion support. For
Homebrew, see <https://docs.brew.sh/Shell-Completion>

Usage:
  gh completion [flags]

Flags:
  -s, --shell string   Shell type: {bash|zsh|fish|powershell} (default "bash")

Global Flags:
      --help              Show help for command
  -R, --repo OWNER/REPO   Select another repository using the OWNER/REPO format