Make node.js compile time support an opt-in feature

[CHH]

People have been reporting that the mere presence of a package.json (for installing Node.js based dev tools and such) increases build times unnecessarily when these tools are not needed when the slug is compiled.

I think we shouldn't install package.json requirements by default, and make this feature opt-in through a switch in composer.json.

[stof]

the other option could be to make these people use the multi buildpack if it works properly (enabling both the PHP and the node buildpacks)