To make it easier to use the plugin, pull in `EEPROMSettings` by default, and
explicitly call its `onSetup` (it is safe to do so), so user sketches don't have
to if they don't use `EEPROMSettings` directly. Also set `Layer.getKey` to
`EEPROMKeymap.getKeyOverride` to provide a sensible default.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>