Added `writerHtmlQTags` and `--html-q-tags` option.

The previous default was to use `<q>` tags in HTML5.
But `<q>` tags are also valid HTML4, and they are not very
robust in HTML5.  Some user agents don't support them,
and some CSS resets prevent pandoc's quotes CSS from working
properly (e.g. bootstrap).  It seems a better default just
to insert quote characters, but the option is provided  for
those who have gotten used to using `<q>` tags.
README on docx styles:  Image Caption, not Picture Caption.