this introduces the PropertyDrawer node to make it easier to access the
properties associated to a headline - normal drawers don't parse their content
into kv pairs
- we can't just look at the len of the string (~ #bytes) - that breaks down for
tables containing characters consisting of multiple bytes. This handles
more (still not all) cases and is good enough for now
- add _ to allowed tag chars - also require space between headline and tags
- links (link itself, not the description) spanning multiple lines are not
supported - otherwise we would have to take care of splitting link and adding
indentation for org pretty printing - and that sounds like such an edge case
that it seems cleaner to forbid them
Until now the footnotes section was parsed but not included in the resulting
AST - his required rebuilding it in the OrgWriter. It feels cleaner to include
it in the AST and only exclude it in the export
