upgrade - add a way to downgrade from prerelease to the latest official version

[Zieleslaw]

I sometimes upgrade my company's package on my PC to prerelease version (for a review etc.) and then I want to bring official version back. I have two options:

• Uninstalling and installing the package again without --pre flag (performs scripts even if I already am on official version),
• Upgrading to specific official version (requires checking version's number).

I would like to have the third option without drawbacks of the two above, going something like:
choco upgrade package-name --allow-downgrade --exclude-prerelease
Current:

package-name v1.0.1-pre is newer than the most recent.
You must be smarter than the average bear...

Expected:

You have package-name v1.0.1-pre installed. Version 1.0.0 is available based on your source(s).

Another flag like "official-only" would also work for me.
Discussion in #686 makes sense to me, but refers to situation when before upgrade there is a package installed that is older than the latest official. In my case, installed package is newer than the latest official.

[Zieleslaw]

As far as I can tell, it's all about this line: https://github.com/chocolatey/choco/blob/develop/src/chocolatey/infrastructure.app/services/NugetService.cs#L745
if (installedPackage.Version > availablePackage.Version && (!config.AllowDowngrade || (config.AllowDowngrade && version == null)))

If we had config parameter RestoreOfficial for Choco Upgrade, it would look like:
if (installedPackage.Version > availablePackage.Version && (!config.AllowDowngrade || (config.AllowDowngrade && version == null && !config.RestoreOfficial)))
Thanks to this change, following line can execute: https://github.com/chocolatey/choco/blob/develop/src/chocolatey/infrastructure.app/services/NugetService.cs#L795