Add option to only parse $ALL_CAPS env vars

[vladnosek]

Especially for nginx configs, lots of directives use $lowercase_variable, so having to escape a lot of content.

It would be nice to have an caps-only flag to only detect env vars that are uppercase.

[dnitsch]

I could jump on this if there are no other takers for a PR on this.

@a8m the only thing I guess is to settle on the bool flag name --no-lower maybe?

[a8m]

Hey @dnitsch and @vladnosek,
I'm fine with adding this option (including the new flag). Please feel free to send a PR and tag me there. Thanks 🙏