Add BuildProgressPlugin

Created by: Timer

I figured I'd take a stab at #935 (closed) since it seemed relatively easy.

I do have a concern though! I created the plugin in react-dev-utils per suggestion in #935 (closed), however this resulted in adding webpack as a dependency 😱 -- it was listed as a peerDependency previously (which fails to resolve post lerna/lerna#318). Would it be better to implement it in react-scripts and add progress as a dependency? Or would you rather explore a solution which doesn't use progress?

edit: read below