tools,doc: fix incomplete json produced by doctool

Doc tool produces incomplete json when it meets unordered lists that
directly following a heading. Add a default case to processList function
to handle the lists.

PR-URL: https://github.com/nodejs/node/pull/5966
Fixes: https://github.com/nodejs/node/issues/1545


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>