Feature request: Allow symbolic names in `install_version`

[dmurdoch]

A recent update of a package on CRAN has caused me some trouble. What would be great is if I could say

install_version("pkg", version = "previous")

to go back to the previous one. (I don't mean the one I previously installed, I mean the latest available version that is less than the one that is currently installed.) This would presumably require a function like available_versions() to find what's available on repositories being searched.

Other symbolic names would be "next", and maybe "latest - n" for some values of n.