Ability to merge YAML settings rather than overwrite

[mbof]

Currently, most settings provided in YAML overwrite the previous configuration (for example, if waypoints or routes or MMSI directories are provided, they erase any previous waypoints or routes).

A feature would allow merging these settings rather than overwriting previous ones.

I'm unclear on what the best way to do this would be. Maybe extra syntax, maybe a modal asking the user what they want to do.