Add support for 'env-file' key.

Signed-off-by: Alan Descoins <dekked@gmail.com>
Use underscores for consistency
Simple test for env_file
env_file should take multiple files
Test that env overrides env_file
Add docs for env_file

Signed-off-by: Ben Langfeld <ben@langfeld.me>
Merge pull request #1 from benlangfeld/support-env-file

Support env file