Mammoth

* Add ignore_empty_paragraphs option, which defaults to True.

* Always use forward slashes in ZIP paths. This should fix image handling on
Windows.

* Make style names case-insensitive in style mappings. This should make style
mappings easier to write, especially since Microsoft Word sometimes represents
style names in the UI differently from in the style definition. For instance,
the style displayed in Word as "Heading 1" has a style name of "heading 1".
This hopefully shouldn't cause an issue for anyone, but if you were relying
on case-sensitivity, please do get in touch.

* Add support for hyperlinks to bookmarks in the same document.

* Add basic support for Markdown. Not all features are currently supported.

* Add default style mappings for builtin footnote and endnote styles in
Microsoft Word and LibreOffice.

* Allow style mappings with a zero-element HTML path.

* Emit warnings when image types are unlikely to be supported by web browsers.