Tiny fix, I'm sure it can be beautified or refactored

Replacing prefix only if is in head of line.

Tested using below
nvm proxy http://username:password@cchproxy:8080 & nvm proxy

nvm proxy username:password@cchproxy:8080 & nvm proxy
nvm proxy http://proxy:10080 & nvm proxy
nvm proxy http://proxy:proxy@proxy:10080 & nvm proxy
nvm proxy proxy:10080 & nvm proxy

concerning  https://github.com/coreybutler/nvm-windows/issues/186

Show message when requesting a 64-bit version that is only installed as
32-bit. Mirroring what is shown when requesting a 32-bit version that is
only installed as 64-bit.

Fixes #90.

load versions from https://nodejs.org/download/release/index.json

fix: 'install latest' works
fix: current version arch works

This is untested because the build chain isn't one I'm prepared to deal with.

download and unzip npm in a temp directory under the nvm root. fix issue
#46