nodebrew install is slow #59

nodebrew install v6.9.1 is too slow because nodebrew just run make && make install.
https://github.com/hokaccha/nodebrew/blob/v0.9.6/nodebrew#L115-L118

I'd like to build concurrently so I suggest we use environment variable like MAKEOPTS or MEKE_OPTS.
https://github.com/rbenv/ruby-build#special-environment-variables

Thanks.