PoC: Pandoc lua filter instead of Beautifulsoup preprocessing

[marph91]

Related: #16

[marph91]

It's possible to migrate some filters to lua, like modifying the image attributes (synology_note_station.lua). I. e. when only tha AST needs to be modified. But for example for tables this doesn't work. If the divs and spans inside tables are not removed before parsing, there is no table at the AST at all. Instead there are plain elements that can't be converted back to tables in a lua filter.