Allow custom build script

[thedjdoorn]

This PR allows a user to add a custom build script (as long as it's defined in package.json). Why is this useful? Well, what if you want to deploy to Pages using a specific environment (not necessarily production)? This PR aims to fix that without having to change the behavior of npm run build

[xRealNeon]

Can you please add the option to the docs in the README.