Allow tables to be made sortable by users.

Uses sorttable. Licensed under MIT by Stuart Langridge -
http://www.kryogenix.org/code/browser/sorttable/

Add support for sortable tables in HTML Publications.

Editor adds {sortable} after any table and all columns become sortable
with a click in the header.

Add tests for adding a sortable table and combining these with barcharts
Merge pull request #763 from ElliotJH/add-sortable

Allow tables to be made sortable by users.