Suggestion: Add ability to merge .env file values with process.env values

[RomaRudyak]

Hi runtime-env-cra team,
Thank you for the tool it helps with a build-once-deploy-everywhere approach.

I would like to suggest merging .env file values with process.env.
The flow:

1. load the .env file
2. the override values from process.env

This will help to have default configs and dynamically override only few of them.

Thanks

[kHRISl33t]

Hey!

Let's say I have a .env.example like this:

FOO=BAR
API_URL=https://something.api.com/

And where the application runs, only API_URL is set in the environment variables, and FOO should be set dynamically from the .env.example because it's not set as an environment variable.

I can see this working with a CLI flag. Something like --dynamic-merge? What do you think?