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.