Display difference from the previous build in filesize

Created by: gaearon

Update: The issue is claimed by @elijahmanor.

Seems like a nice thing to do. We can read the files before clearing the build folder.

It could look like this:

Creating an optimized production build...
Compiled successfully.

File sizes after gzip:

  48.73 KB (+2.12 KB)  build/static/js/main.42e82b08.js
  289 B                build/static/css/main.9a0fe4f1.css

(Right now we display the build size but not the delta.)