Allow updating an instance automatically if needed

[kzu]

It's a bit annoying to have to run the VS installer just to check for updates (or even if the updates are already downloaded) and install them.

Would be nice to come to the computer every day knowing that any updates have already been applied overnight.

Proposed syntax after talking to @adalon is:

vs update [nickname] [options]

Options could be the same ones we currently have for locating VS instances, such as --sku=[ent|pro|com] and --pre|preview or --int|internal. We currently have a --nick|nickname= argument for vs install. It's an optional argument there, but it might be worth keeping things consistent. Not totally sold on passing arguments as "simple" strings without an argument name, since that makes them harder to discover and document too (at least with Mono.Options built-in rendering which helps a lot out of the box).

Examples pf proposed syntax:

vs update master
vs update stable
vs update all

[kzu]

Done by @adalon