util.human.io: Allow defining per column ellipsis function

As an alternative to doing it in the mapper function. Could be useful in
cases where one may want to put the ellipsis in the middle or beginning
instead of the start.

util/human/io.lua

@@ -176,7 +176,7 @@ local function new_table(col_specs, max_width)
 					v = padright(v, width);
 				end
 			elseif len(v) > width then
-				v = ellipsis(v, width);
+				v = (column.ellipsis or ellipsis)(v, width);
 			end
 			table.insert(output, v);
 		end