Update the nvm itself

[i-curve]

Hello, I am a user of this repository.
I found that nvm doesn't have update itself function, I think it's useful to be able to update itself.

Two functions related to this feature:

nvm update: to detect if a new nvm version can be installed.
nvm upgrade: to install the newest nvm version or specific version.

I wonder if nvm needs to support this feature or is already in development.
If the feature is needed and hasn't been developed yet, I hope I can contribute it.

[ljharb]

Duplicate of #400.