Responsive table should support multi-rowed headings

Created by: meeprophone

It's common on our pages to have two rows within the thead, where the top row comprises grouping columns with a colspan. For example:

+-------------------+-------------------+     }
|        2013       |        2014       |     }      thead
+------+------------+------+------------+     }
| Lent | Michaelmas | Lent | Michaelmas |     }
+------+------------+------+------------+
| Blah | Blather    | blah | Blather    |     }    tbody
+------+------------+------+------------+     }

This breaks in the mobile layout, but in principle it's possible to discover all the relevant headings and list them as field headings. (This should work also for the rare, but not unknown, 3-level theads)

2013: Lent Blah

2013: Michaelmas Blather

2014:, Lent blah

2014: Michaelmas Blather

This enhancement would make the responsive tables more useful.