arg handling made simpler for now

Raguel has issues with nesting if_* logic syntactical sugar, shall be
fixed in its own time and adapted back.
Till then if required, will add crude nesting.
added multiple md-to-html converter support

Right now just supports Pandoc extra, check for it's existence if not
found... falls back to Github converter (also in case of wrong converter
name); default converter though is still Github.
To switch converter, set "DATUM_CONVERTER" env var to "pandoc" for
pandoc or other when there is support.