Add compatibility with yarn

[taskbox-joe]

This makes it so you can add a parameter in your Fastfile that will make this plugin work with yarn. I think that it may only be required for yarn 2+ as yarn 1 still packages everything in to node_modules folder whereas this may not be the case in yarn 2.

Hint: I have never done any Ruby development before so 🤞 I haven't made any stupid Ruby specific mistakes that I am not aware of