Environment variables meet macOS Keychain and gnome-keyring <3
Linux support with libsecret (#7 @eagletmt)
libsecret commuicates with D-Bus secret service; for instance, gnome-keyring implements it.
--list
command
On OS X keychain, values have to be set after envchain v0.2.0 or later due to data structure change, in prior to be listed.