WIP: Allow to append default mapping file

[olivierkeke]

This PR propose a method to let the user update default mapping.yaml without copying the entire file.

User must add an extra_mapping_file variable in the enoceanmqtt.conf file. This extra_mapping_file is a yaml file with similar structure than mapping.yaml file. Removing an entity is done by adding action: remove in the entity description.

[OlwinFroon]

Now THAT would be most helpful 👍