Add --silent option to build command (#5029)

Created by: lukasgjetting

This adds a --silent option to the react-scripts build command.

When using the --silent option neither file sizes nor hosting instructions will be logged.

This is useful when building remotely, as the current amount of "unnecessary" information can really clog the output.

Test plan

1. Run react-scripts build and see that it prints the same information as before

$ yarn build
yarn run v1.6.0
$ react-scripts build
Creating an optimized production build...
Compiled successfully.

File sizes after gzip:

  207.02 KB  build\static\js\1.6875191b.chunk.js
  12.55 KB   build\static\js\main.d7e743ce.chunk.js
  2.76 KB    build\static\css\main.fba159f8.chunk.css
  2 KB       build\static\css\1.5d61c275.chunk.css
  763 B      build\static\js\runtime~main.229c360f.js

The project was built assuming it is hosted at the server root.
You can control this with the homepage field in your package.json.
For example, add this to build it for GitHub Pages:

  "homepage" : "http://myname.github.io/myapp",

The build folder is ready to be deployed.
You may serve it with a static server:

  yarn global add serve
  serve -s build

Find out more about deployment here:

  http://bit.ly/CRA-deploy

Done in 67.75s.

2. Run react-scripts build --silent and see that it doesn't print the information specified above

$ yarn build:silent
yarn run v1.6.0
$ react-scripts --silent
Creating an optimized production build...
Compiled successfully.

Done in 17.63s.