* new slog in stdlib is utilized for that and replaces 'log' pkg
* configuration pkg is used validate and now set/define/refer
  log level values
* the 'error' level call is not fatal, thus os.Exit is called
  instead

* rework indentation of some configuration variables
* rename some 'err' variables to '_' since they are not used

Available levels for now: debug, error. While the former is the
most verbose one and the latter the least verbose.

It sets it to 'debug' if in development mode.

The version is displayed on the HTML-based landing page.

This is being handled by Fiber, hence not tested.