Crash the build during CI whenever linter warnings are encountered

Created by: excitement-engineer

Fixes issue #906 (closed).

Whenever someone runs the build script with the environmental variable CI set to true then it will crash when any linter warnings are encountered.

CI=true npm run build

I have tested this locally by generating an app using create-react-app and the updated react-scripts. Is this enough or should there be an automated way of testing this functionality? If so, can anyone give some advice on how to go about testing this?

I am also struggling to find the best location to document this feature. I considered updating the npm run build section of the README with this information however, I am afraid that explaining this feature in that location will only detract from the clarity to the reader. Perhaps I should create a new section in the user guide where this is explained, any thoughts?