Mammoth

* Throw more informative error when word/document.xml cannot be found.

* Generate messages of type "error" instead of "warning" when image conversion
throws an exception.

* Use alt text title as alt text for images when the alt text description is
blank.

* Handle comments without author initials.

* Change numbering of comments to be global rather than per-user to match the
behaviour of Word.

* Add support for comments.

* Add support for w:sdt elements. This allows the bodies of content controls,
such as bibliographies, to be converted.

* Avoid stack overflows when elements have many children.

* Actually add support for table cells spanning multiple rows.

* Add support for table cells spanning multiple rows.