Support updating a single package

[Wilfred]

It would be really useful to update a single package. I don't like upgrading everything at once, because things break without me noticing.

Currently, I'm sorting based on package name, then marking the latest version for installation, and the old version for removal. This is already an improvement on package.el, but would you be willing to add upgrade as a feature?

[Malabarba]

I'd take that as a PR. Shouldn't be too hard to implement a command like U which applies only to one package.