Getting linearity to reveal fragments. This avoid rendering of - (none) cells before fragments.
Fixed restarting cell to (-) type. Added a final point to the while cycle.
Updated reveal CDN and use the new data-fragment-index attribute to get composite fragments (several cells inside the same fragment).
Fixed test and add fragments to the list of resets.
Merge pull request #5957 from damianavila/fragments

IPython slides: fragments are now in a linear flow and include next "-" cells.