Add `nvm_get_latest`
Tests for `nvm_get_latest`
Merge pull request #599 from ljharb/nvm_get_latest

Add `nvm_get_latest`

This relates to #522, #561, #400.

`nvm_get_latest` uses http://latest.nvm.sh which redirects to https://github.com/creationix/nvm/releases/latest which in turn redirects to the latest release tag.