* Implement GetArmoredWithCustomHeaders
ArmorWithTypeAndCustomHeaders can be reused by other PGP armoured
objects.
* Update linting, and lint accordingly
`godot` has been improved and `goerr113` has been added (and ignored
here).
* Add custom headers for keys
* Minor comment changes
Co-authored-by: Aron Wussler <aron@wussler.it>