Adjust Pelican Makefile & configuration before future upgrades

[atodorov]

As part of the redesign Pelican was updated to v4.2.0, see 227fe0f, which now causes various warnings:

$ make serve:

WARNING: 'python -m pelican.server' is deprecated.
  | The Pelican development server should be run via 'pelican --listen' or 'pelican -l'.
  | This can be combined with regeneration as 'pelican -lr'.
  | Rerun 'pelican-quickstart' to get new Makefile and tasks.py files.

$ make regenerate
pelican -r /home/senko/kiwitcms.github.io/content -o /home/senko/kiwitcms.github.io/output -s /home/senko/kiwitcms.github.io/pelicanconf.py 

WARNING: SLUG_SUBSTITUTIONS and variants thereof are deprecated and will be removed in the future. Please use SLUG_REGEX_SUBSTITUTIONS and variants thereof instead. Check http://docs.getpelican.com/en/latest/settings.html#url-settings.
  --- AutoReload Mode: Monitoring `content`, `theme` and `settings` for changes. ---

These would probably break if we decide to upgrade to v5 in the future.