Automate SLDS CSS setup in Create React App

[interactivellama]

Automate the following: https://github.com/salesforce/design-system-react/blob/master/docs/create-react-app.md to enable one CLI line setup something like:

npm install @salesforce/design-system-create-react-app-setup

to setup SLDS CSS usage in Create React App (via npm script postinstall probably?). I would think that you if ran this again, it would copy it again.

Although it might make more sense to put this in the @salesforce-ux org. @amyleesalesforce do you have any thoughts? This doesn't really involve Design System React. It's just helpful for any React/SLDS app--and maybe some of your all's in the future.

[interactivellama]

Another option would be to fork react-scripts. If we did a fork, this would be nice to keep them up to date. https://github.com/backstrokeapp/server