Make .nvmrc more versatile/modular

[victormihalache]

Made because of #3234.

As of opening this PR the only difference is that a new --config= flag can be used to indicate the path to the .nvmrc (or any other name for that matter) to be used specifically. It is available to all commands that utilize the .nvmrc file to begin with, those being: install, use, exec, run, which. I have tested all of them using the docker container provided.

What I want to add next are the following features:

• Look for an .nvmrc file if the path is to a folder, rather than a file.
• Add more places to look for a config file, besides the .nvmrc in the root.

I would also love to hear the thoughts of @ljharb and other maintainers, but also general users of nvm!

[ljharb]

The PR was premature pending discussion on the issue; I don't value "less config files in the root", and thus the added complexity to nvm is unlikely to be worth it to me (the only maintainer). Let's continue discussion on the issue.