Feature Request: env -f to read from file

[Balakrishnan Balasubramanian]

When env command is used to pass extra environment variables to a 
program, it will be useful to read them from a file instead of command 
line.

Instead of

    env MYAPP_QA_KEY=XXXXX MYAPP_QA_URL=https://qa.myapp.example.com 
clicommand

We can use

    env -f qa.env clicommand

This way command-line can be kept simple and easy to change to different 
environments instead of sourcing files.

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Regards,
Bala