RFC: allow specifying particular packages in Pkg.update

This adds a single package form of `Pkg.update`, which can be invoked as:

```
julia> Pkg.update("FooBar")
```

to get new versions of FooBar *only*. You can also do `Pkg.update("METADATA")` to just update METADATA without upgrading packages. I am somewhat unclear on what's going on with all the cache prefetch stuff and this duplicates a lot of code. There's probably a better way to factor this, but it seems to work so I figured I'd open an RFC just to get discussion going if nothing else (there's been some mailing list threads lately about this functionality and it's something I frequently want myself).