We want original whitespace to be rendered in some cases (e.g. verse
blocks). This requires information about the original whitespace to be
preserved during paragraph parsing. As html ignores (collapses) whitespace by
default we don't have to adapt the html writer and can just selectively enable
rendering of the preseverved whitespace wherever we want it using
css (white-space: pre).
To differentiate meaningful whitespace from document structure based
indentation (i.e. list item base indentation) we need to introduce
document.baseLvl. A paragraph by itself does not have enough information to
differentiate both kinds of whitespace and needs this information as context
[0].
As we're already touching list indentation i went along and improved (fixed?)
descriptive list item indentation rendering in the org writer (it should match
emacs tab behavior - i.e. indent subsequent lines up to the `:: `).
[0] e.g. list items can contain blank lines - a paragraph starting with a blank
line would not know that it is part of a list item / has a base indentation -
the blank line would suggest a baseLvl of 0.
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.
