Allow opting into pre-releases per-package

[jtackaberry]

Currently the feature to automatically install prereleases is a global setting. This should also be available on a package-by-package basis, overriding the global setting.

Because of this, Reaticulate distributes both a release ReaPack and a prerelease ReaPack, so that users on the prerelease track automatically receive updates. But if ReaPack provided the ability to enable prereleases for specific packages only, rather than globally, I could consolidate these ReaPacks.

[cfillion]

Installing a pre-release version in the package manager automatically opt-in the user for subsequent pre-release updates of the package. The opt-in is active up to the next stable version.

[jtackaberry]

The opt-in is active up to the next stable version.

And that's the crux of this FR: to be able to opt in to the latest version of a package in perpetuity, even if it's a prerelease, as part of the automatic upgrade process.

[cfillion]

Done, here's a test build (backup registry.db first): https://ci.appveyor.com/api/buildjobs/i4xdsqsx88bph2i3/artifacts/build%2Freaper_reapack-x64.dll.

[jtackaberry]

Wow, that was fast!

@cfillion I tested out all the scenarios I could think of related to moving between releases and prereleases, and everything looks good. to me. 👍

Thanks!