WriteNodesAsString is simple enough to implement but exposing it is helpful in
the implementation of extending writers and we don't aim to keep writer a small
interface so let's expose it.
Being able to very easily get the original [1] Org mode content seems like
something that will come up quite often and is very little code.
[1] it's not really the original content, but rather the pretty printed version
of that - as the semantics don't change it shouldn't matter.
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
- drawer entries without value were printed as FOO rather than :FOO:
- account for differences between raw & non-raw block:
raw blocks are not wrapped in a further element, just raw text & line breaks:
-> the first line has to be indented manually
non raw blocks do not end in a linebreak newline -> the END_BLOCK line has to
be indented (rather they end with a manual newline from another element)
