Expose command to set entries for keyring transform

[VorpalBlade]

The recent breakage in hwchen/keyring-rs#207 made me realise that recommending using secret-tool to set the user and password probably won't work on Windows and Mac. A sub command should be added. However hwchen/keyring-rs#208 means that the obvious way of doing this isn't great.

• Find a way where the label isn't bad (Suboptimal label when using the platform independent API with secret service backend hwchen/keyring-rs#208)
• Implement sub commands to set / remove key
• Update docs to describe this new method of setting the password.
• Find migration path (ideally transparent so we don't have to do a breaking change for this) for existing passwords, that is not platform specific.