Ventcrawl port from Bay12/VG (#852)

Original code from /vg/, port credit goes to Zuhayr.

code/modules/ventcrawl/ventcrawl_verb.dm

@@ -0,0 +1,9 @@
+/mob/living/proc/ventcrawl()
+	set name = "Crawl through Vent"
+	set desc = "Enter an air vent and crawl through the pipe system."
+	set category = "Abilities"
+	if(paralysis || restrained())
+		return
+	var/pipe = start_ventcrawl()
+	if(pipe)
+		handle_ventcrawl()