Make check channel up-to-date in julialauncher optional

[torfjelde]

As mentioned in #705, the fact that juliaup always prints when an update for the channel in use is available, can sometimes break scripts, etc. which rely on parsing outputs from Julia scripts since this message is also printed to stdout.

This PR adds an option to the config which allows the user to specify whether or not to print the aforementioned warning/info.

I don't really program in Rust, and this was the first time in ages, so I probably didn't implement everything correctly :grimacing: There might also be an easier way to all of this, but I figured I'd just give it a go.

[tamasgal]

Could we merge this, or does it need more work?

[davidanthoff]

Wouldn't a much easier solution to this problem simply be to print the update message to the stderr instead of stdout?

[davidanthoff]

Actually, lets discuss in https://github.com/JuliaLang/juliaup/issues/705.