Provide fallbacks for parser config settings.
Some 3rd party applications fail with AttributeError due to
missing values in the document.settings object.
a) because they don't follow the requirement documented in
utils.new_document:
If you will use the document object with any Docutils components,
you must provide their default settings as well.
b) because they may provide a hard-coded set missing new settings.
This should fix bug #415 and
https://github.com/readthedocs/recommonmark/issues/220.
See also the sphinx issue with Docutils 0.13.1
https://github.com/sphinx-doc/sphinx/issues/3951.
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@8671 929543f6-e4f2-0310-98a6-ba3bd3dd1d04