Passphrase from env

[dickon]

This is useful when redirecting stdout to a file.

[sommer]

Good suggestion! In general, I like the change, but I'd rather merge this without the change to the README file. The new text seems to imply that automating opening the safe via environment variable is less safe than opening it by other means. In fact, if what a user is looking for is to open a vault programmatically, using an environment variable is likely safer than most options one could come up with.