Keep nvm_prefix if set

[ribugent]

Currently, I was dealing with nvm on my Arch Linux installed from AUR. The package manager install it in /usr/share/nvm/

The plugin didn't work well, setting NVM_DIR ended up nvm trying to download and install nvm version in /usr/share/nvm/.

Looking in the code, I discovered that there's a patch for brew users that sets the nvm_prefix for them. So I'm proposing on this patch allow the users to set their own nvm_prefix for similar cases.

Hope this helps