Feature request - allow setting of master pasword via environment variable

[dannyfast]

At present either a password must be typed in or an unencrypted file must be present to encrypt a realm.

If the password could be specified with an environment variable the command requirements could be cut by 50% and concerns over master file storage security would be removed.

[con-f-use]

Depending on your use-case, you might want to do something like:

decrypt-command crypted-master-pw | gokey -P /dev/stdin -r test
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But yeah, environment variables are a pretty standard interface, that gokey should support. Same as -P - i.e. a dash character instead of a file name, to read the master-password from standard input.

[ignatk]

Done in #72