try to solve issue #2 - capability to set a class 'no-toc-progress' on a section element

with this patch,
when a section (secondary section) has a no-toc-progress class
his title it's excluded from the toc footer
AND
when displaying this slide the second column of the toc footer does not disappear as shown in comments of issue #2
Merge pull request #3 from niconil/patch-1

solve issue #2 - capability to set a class 'no-toc-progress' on a section element