Mammoth

* Support links and images in footnotes and endnotes.

* Add support for underlines in style map.

* Add support for strikethrough.

* Add basic support for text boxes. The contents of the text box are treated as
a separate paragraph that appears after the paragraph containing the text box.

* Support styles defined without a name

* Add ignoreEmptyParagraphs option, which defaults to true.

* 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.