New configuration layer, minimum PHP 7.2, lots of bugfixes

Highlights

• Drop PHP 7.1 support, minimum is PHP 7.2 now
• Rewrite the configuration layer to be typed, fixes a lot of edge cases, now allows changin "serve" options through the command line and simplifies a lot of code paths #171

Bugfixes

• Fix relative links for images, now works correctly in serve and generate mode #169 #78
• Fix bug where files with non-ascii names would be blank, they are now transliterated #183
• Fix Nesting level too deep - recursive dependency? when multiple files have the same name #182

Internal

• Fix a type issue in an update #181
• Lazy load the CommonMark converter, only loaded when we actually need to generate a page, wins around 10ms