Move webpack to peerDependencies

[yesmeck]

Since this plugin support both webpack 3 and 4, webpack should add as a peer dependency.

[NekR]

Hi @yesmeck! Thanks for making this PR, it helped me find that all the dependencies were messed up die to npm 6's auto-adding dependencies on npm install.

I'm going to close this since there were more dependencies issues and I had to fix them myself, but many thanks anyway!

I've a new release already 👌