Add preflight check to guard against wrong versions of webpack/eslint/jest higher up the tree

Created by: gaearon

It looks like this:

I thought about extracting it to gist but I kinda like that we can substitute the dep name right into the message. We might also want to do something similar for duplicate React.

There is an opt-out mechanism mentioned at the very end (cc @jlongster):

SKIP_PREFLIGHT_CHECK=true

in .env file.

"Fixes" https://github.com/facebookincubator/create-react-app/issues/1795.