`nodebrew install` is slow

`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.
