Docx Reader: All headers get auto id.

Previously, only those with an anchor got an auto id. Now, all do, which
puts it in line with pandoc's markdown extension.
Update docx tests to reflect new expected behavior

Now doing auto ids for all headers, so tests should reflect that.
Merge pull request #1379 from jkr/more_auto_id

Docx Reader: Extend auto id behavior to all headers
Docx Reader: Update state properly

Previously, a fresh state was created for the purpose of updating. In
the future, when there is more than one field in the state, this
obviously won't work.
Merge pull request #1381 from jkr/updateDState

Docx Reader: Update state properly
Docx writer: insert bookmark tags inside <w:p> tag.

This makes the header anchors in pandoc-generated ooxml match those
generated by word.
Merge pull request #1383 from jkr/writer-bookmark-fix

Docx writer: insert bookmark tags inside <w:p> tag.