docx.batch/2: allow tables (and nested blocks) in header/footer stories

[bobzhang]

Follow-up to #95 / #352.

The WRITER API accepts tables in a header/footer story — check_story_body
allows Paragraph and Table, and docx2html/write_headers_test.mbt pins a
header containing a table that validates.

The docx.batch/2 JSON surface does NOT: a header/footer op takes text xor
paragraphs[], mirroring a comment body. Report headers with a two-column
layout table are common, so the grammar should grow a block list, e.g.

{"op": "header", "params": {"blocks": [{"table": {...}}, {"paragraph": {...}}]}}

exclusive with text/paragraphs. Kept out of #352 to hold the PR to a
reviewable size.