Docx Reader: Add change types

Insertion and deletion. Dates are just strings for now.
Docx reader: Parse Insertions and Deletions.

This is just for the Parse module, reading it into the Docx format. It
still has to be translated into pandoc.
Docx reader: Add rudimentary track changes support.

This will only read the insertions, and ignore the deletions.
Docx reader: Remove unnecessary filter in Parse.

mapMaybe does the filtering for us.
Docx reader: Add tests for basic track changes

This is what seems like the sensible default: read in insertions, and
ignore deletions. In the future, it would be good if options were
available for either taking in deletions or keeping both in some
scriptable format.
Merge pull request #1372 from jkr/track-changes

Rudimentary track-changes support