Use a paragraph in table header of extra columns of item-matrix
When using :sourcecolumns:
or :targetcolumns:
of the item-matrix
directive in v8.3.0, some table headers in HTML format did not contain a paragraph element, which could result in an undesired layout of the text. This release fixes that issue. All table headers of a rendered item-matrix should contain a parapgrah element. [PR #245]