Honor .node-version file

Forgive me if this has been discussed before, but I didn't find anything beyond https://github.com/creationix/nvm/issues/110#issuecomment-40525629.

It would be really nice to see the various Node version managing tools coalesce around a common "version config file", analogous to Ruby's .ruby-version for RVM and rbenv - .node-version. This would basically just be an alias for .nvmrc, since that functionality's already been added. Since n already supports .node-version, this would make it so that regardless of the Node version manager used, there could be some degree of guarantee that the correct version will be used.