[feature request] update package to possible newest version while keeping least copy
I don't know whether I missed something in document. I am saying, is it possible to update pacakge and its supporting pacakge(s) to possible newest version, and delete unnecessary old version automatically via, for example `nimble update xx`?

I know
1. we can use `nimble uninstall xx@1.2.3` to remove xx version 1.2.3 by hand. But I mean if we do `nimble update xx` then `nimble` removes all old version by itself. Currently, I find there may be multi-version of some pacakges on my windows, so I have to remove them by hand.

2. some old package is used by others, so `nimble update xx` does not remove the old one.

I hope I have stated it clear. Thanks
