Refactor Org mode front matter: Introduce '#+KEY[]:' array notation

Refactor Org mode front matter: Introduce '#+KEY[]:' array notation

Hugo requires some front matter values to be arrays (e.g. for taxonomies).
Org mode front matter syntax (`#+KEY: VALUE`) does however not support anything
but string values normally - which is why goorgeous hardcoded the values for
the keys tags, categories & aliases to be parsed as string arrays. This causes
problems with custom taxonomies.

A simple thing we can do instead is make keywords ending in '[]' be parsed as
string arrays.