Add dependencies of _app.js to commons (#4396)
Add dependencies of _app.js to commons (#4396)

Since `_app.js` is used on every page it makes sense to move it's dependencies to the `commons.js` so that if you require a dependency in `_app.js` and in one of your pages it is not included twice in your bundles. 

This PR modifies the `CommonsChunkPlugin` to move every module that is used in `_app.js` and at least in one other bundle.
zeit/next.js,flybayer/next.js,azukaru/next.js,azukaru/next.js,zeit/next.js,zeit/next.js,BlancheXu/test,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,BlancheXu/test,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,BlancheXu/test