Org reader improvements

These patches make the org-mode reader ready for prime-time &ndash; or at least they make it suck a little less.

Org-mode support is far from complete, but all major features are covered.  Took a little less time than expected.

Remaining issues include:

  - in-document links and anchors are not supported.
  - extra link attributes (via `#+HTML_ATTR`) are not recognized.
  - same for block attributes.
  - `#+include` statements are not handled (new `Include` block type-constructor?).
  - code block arguments are not parsed, hence there is no way to execute code blocks from within a filter.

Please let me know if any of those must be considered a show stopper.