Fix: move index.js and configs

[mysticatea]

It have thrown "Module not found" error when I do require("eslint-plugin-vue") with 3.0.0 because the package.json has main: "lib/index.js" field and the lib/index.js does not exist.

This PR moves files:

• index.js → lib/index.js
• Incidentally, config → lib/config because I think that the config directory should exist at the same level as rules directory.

[michalsnik]

Ah, right. I thought eslint-generator puts index out of lib directory by default so I moved it. Thanks for catching this!