When we put functionality into submodules, it doesn't make sense to have one monolitic configuration file with options which may or may not apply to the modules installed.

Switch the configuration file scheme to have a core configuration file (probably /etc/taskotron/taskotron.yaml) in addition to loading module-specific configuration files from a directory (probably /etc/taskotron/conf.d).

This also includes making sure that config defaults work when they may or may not be part of a single file.