Fixes syntax errors for columns and visibility

I've added some commas missing from _visibility.scss that were causing certain visibility classes to be unavailable. Also fixed an issue with modifying loops in @mixin grid-html-classes(), where starting the loops at 0 would yield classes like ".small-push0" instead of ".small-push-0". This is due to the hyphen being inside #{-$i}, which evaluates negative 0 as 0. Moving the hyphen outside of the braces restores the hyphen. (And from a pedantic standpoint, the hyphen is used as a hyphen in the class name, not as an indication of negation.)