Mammoth

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

* Add support for table cells spanning multiple rows.

* Add support for table cells spanning multiple columns.

* Improve script installation on Windows by using entry_points instead of
scripts in setup.py.

* Remove deprecated convert_underline argument.

* Officially support ID prefixes.

* Generated IDs no longer insert a hyphen after the ID prefix.

* The default ID prefix is now the empty string rather than a random number
followed by a hyphen.

* Rename mammoth.images.inline to mammoth.images.img_element to better reflect
its behaviour.

* Improve collapsing of elements when there are empty elements.