Add support for env files

I'd suggest the following enhancement of envsubst to be more useful in combination with Docker/Docker Compose or Systemd Unit files:
* Add option to read environment from env files
* Allow multiple env files to be given and process them in order (to override values from former files to support development purposes)
* Add option to ignore the existing shell environment variables and build a fresh environment using the given env file(s)
This would greatly simplify some development and deployment processes.

Look at the main command implementation in cmd/envsubst/main.go to understand the current argument parsing and substitution logic. The feature requires adding new flags for env files and possibly a fresh environment mode. Check how Go's os package reads environment variables and consider using a library or writing a parser for env file format. Test by building the tool and trying it with sample env files.

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