Add BuildProgressPlugin

[Timer]

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

I do have a concern though!
I created the plugin in react-dev-utils per suggestion in #935, 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

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[Timer]

Using the same technique used in https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/config/paths.js#L74, I changed webpack back to a peerDependency in ee9ca5e.

[Timer]

Closing this as the solution doesn't seem appropriate given the webpack peerDependency was removed in #963 / 9d53d22.
It seems the solution for this will be implemented elsehow (not in react-dev-utils, but perhaps in react-scripts).

[gaearon]

Sorry, I had no time to look at this yet. I will, can you please reopen?

require('webpack').ProgressPlugin should be fine. Peer dependency was only removed because now that we declare compat with Webpack 2, it will be annoying to bump it every few weeks. If npm provided a way to declare compat with all beta versions, I would've kept it.

But we are assuming specific plugins in react-dev-utils rely on Webpack. It's fine to import anything from it if these things are supported in 1.x and 2.x.

[Timer]

@gaearon Sure, unfortunately github won't let me reopen this pull request as I rebased before reopening it -- oops! I've created a new one (#1011) and added associated discussion to it.