Add $NVM_NODE_PREFIX to override 'node-' prefixes

[kriswill]

ability to replace tar file name prefixes with 'iojs-' when reading
dist listing from https://iojs.org/dist/

example in .bash_profile:

if [ -f ~/.nvm/nvm.sh ]; then
  source ~/.nvm/nvm.sh
  export NVM_NODEJS_ORG_MIRROR="https://iojs.org/dist/"
  export NVM_NODE_PREFIX="iojs"
fi

you can then:

→ nvm ls-remote
      v1.0.0
      v1.0.1

→ nvm install v1.0.1
... long compile time ...

→ type node
node is /Users/kris/.nvm/versions/node/v1.0.1/bin/node

→ node -p process.versions
{ http_parser: '2.3',
  node: '1.0.1',
  v8: '3.31.74.1',
  uv: '1.2.0',
  zlib: '1.2.8',
  ares: '1.10.0-DEV',
  modules: '42',
  openssl: '1.0.1k' }

[ljharb]

I'm already working on iojs support, and this won't be the way I do it. There's far more to compatibility than reading from iojs.org - including that node has SHA-1 sums, and io.js has SHA-256 sums.

Thanks for your contribution though!

See #590.

[kriswill]

Thanks @ljharb. I know it's a lame attempt, I tried to keep the scope small, since I figured there was more you wanted to do with it.