Adds nvm update command

[ljharb]

I'm not sure how to write tests for this - I'd love some guidance.

[creationix]

I'm not sure how this is a useful feature. If nvm works for you, why would you ever want to upgrade to a new version that potentially breaks things? If there are new features and you need them, how much work is it to manually upgrade nvm with a one liner (or git pull)?

Not saying it's a bad idea of bad code, just not sure it's useful enough to justify yet another command in nvm. Simplicity is a good thing.

[creationix]

Though one technical issue. This code assumes you installed nvm with the install.sh script and that re-running that script will upgrade. This is not the case for many people. I'd rather not have a command that breaks things for a lot of people when run.

[ljharb]

• because many other things can upgrade, like your shell, or node, etc, and can break things. It's always a bad idea to lag behind the latest version on anything imo.
• Your point about the install.sh assumption is well-founded. I'll close this for now, but I'll think about a way to do it.

Perhaps a simple command that provided the one-liner(s) would be helpful? I often don't want to have to load up the webpage just to update things.