distinguish between optional, recommended and required configuration settings

[trz42]

The function check_required_cfg_settings(REQUIRED_CONFIG) verifies if all settings listed in the dictionary REQUIRED_CONFIGS are provided in the bot configuration app.cfg. At the moment it can only distinguish between optional settings (those would not be listed in REQUIRED_CONFIG) and required settings (those that are listed in REQUIRED_CONFIG).

There are, however, settings which fall into a third category of recommended settings. The bot will work without them being set, but they are very useful. Because of that (being very useful and not being able to distinguish recommended settings), in #266 they are added to the list of required settings in REQUIRED_CONFIG.

If the check_required_cfg_settings function supports recommended settings, the check at the start of the bot components could make use of that. For example, it could show a warning or hint that some recommended setting is missing.