inject globals from environment variables

[okwolf]

Common use cases for this include logic that's conditionally run depending on environment and API access keys that you don't want to hardcode. We should always support NODE_ENV and possibly any variables that start with a configurable prefix like create-react-app has REACT_APP_.

To implement this, I'm thinking add a <script> that sets window.process.env = { ... } instead of trying to parse and replace the usages of the variables in scripts. It might actually be more helpful during development to see the original code and look at the values with the devtools.