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.
