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.
also dismiss implementing ordered list bullet overrides for now, e.g.
1. [@10] foo
add it once someone needs it - for now it seems like needless complexity
see https://orgmode.org/manual/Checkboxes.html
We're deviating from Org mode regarding the assigned css classes but the chosen
classes feel better than (on, off, trans) and it's not like the export matches
1:1 otherwise.
