Option To Install LTS

By default, this installs node 5.12.0 just fine, but what I want is for it to install LTS (currently 6.9.1). Even when I specify the desired version, it still installs 5.12.0.

I was able to get around this by running `nvm install 6.9.1` after running `node-reinstall`, but it would be nice to not have that additional step.

Ideally, it would be `node-reinstall --lts` or something similar.