New CA_THOK effect

I asked the devs if I could push this to vanilla and it seemed to get a generally positive reception, so here it is. I figure I should credit Krabs for his Rebound Dash script, which I based this off. If somebody could debug that g_demo.c code for me, that would be appreciated. I'm not exactly an expert when it comes to that. I was also considering making the sprite use THOKB0 instead of making a whole new freeslot, but I figured it wouldn't hurt anything being a separate one. Otherwise, please let me know if I screwed anything up with my sloppy coding.

Since the graphic doesn't exist in vanilla, you'll have to download this file: thok_effect.wad