✨ subcommand for automatic updates

[jeftadlvw]

As soon as some sort of an automatic build system is ready and operational, it would be greate if git-nest would utilize it and update itself if the user allows it.

The suggested subcommand would be

git-nest update

Though, maybe this subcommand could be confused with sync, so

git-nest version --update

would be a viable option too.

Because git-nest is just a single executable, it could easily be downloaded, verified and replaced.

This issue requires #17, or some form of consistent binary provisioning for each new release.

[jeftadlvw]

First version of an autoupdater would just look if a new release and binary exists for the local os and architecture, then download and replace the executable.

Next steps could enclude some form of automatic building:

• locally using the installed go toolchain
• locally using the available docker installation