Add options for sorting tables to plonematch theme

Signed-off-by: Sebastien Besson <seb.besson@gmail.com>
supported-formats: sort ratings columns properly

This necessitated the addition of alt tags on the ratings images
(unfortunately, reStructured text does not support title tags),
as well as a hack to sorttable.js to respect them.

Signed-off-by: Sebastien Besson <seb.besson@gmail.com>
Merge pull request #1 from sbesson/sorttable

Restore sortable option in official sphinx_theme