Feature: Ability to downgrade a package

[sammcj]

It would be great if there was an option to downgrade a package.

Reason: This morning I upgraded a package and broke my theme and stopped several other packages from working, it's just a bug in this new version and if I could easily go 'Downgrade Package -> Select Package Name' all would be fine.

I've hit this or similar a couple of times and think it would be a useful feature.