Add sarc/sarcasm/abbr/strike/sup/sub tags

With the nesting fix for sup/sub we can go ahead and allow them. Added encoding to just show the tags if you use sarc or sarcasm tags. The rest don't present any issues that I can find and aren't overly distracting, so we might as well allow them.

Finally, added a new var (approvedtags_visible) to display instead of approvedtags on the pages so we can stealth-add tags and/or keep the list from being too cluttered by removing some of them from display.

Cheers to thors and the #366 pull request for doing half the work.