Feature Request: Add an auto-update mode

I'd like to see a `juliaup` command which would activate an auto-update mode, so casual users can set it and forget it (at least until Julia 2.0). This would check for a new version periodically, delete the outdated version #53, and link the `julia` command to the new version.

I am currently having coworkers install [Chocolatey](https://github.com/JuliaLang/julia/releases/tag/v1.6.1) and the `choco-upgrade-all-at` package to achieve this, but it would be much nicer to have a Windows Store product for ease of finding and permissions.

It would be cool if the auto-update part could take advantage of #43 and `startup.jl` to update whenever a user starts Julia. If not, a cron-like shell script running once a day could also work.

Read issue #43 and inspect the startup.jl path mentioned in this request, then compare it with the proposed cron-like shell script and Windows Store approach. Done means an agreed auto-update path can periodically find a new Julia version, remove the outdated version referenced by #53, and relink the julia command.

Written by the indexing model from the issue text.