Accessibility: showdamage option

Add a new boolean option showdamage, if on, outputs a message
like "[HP -2, 14 left]" - several variants have something similar,
but I chose the message based on how eSpeak said it, while keeping
it short.

src/mhitu.c

@@ -1810,11 +1810,13 @@ mdamageu(struct monst *mtmp, int n)
     disp.botl = TRUE;
     if (Upolyd) {
         u.mh -= n;
+        showdamage(n);
         if (u.mh < 1)
             rehumanize();
     } else {
         n = saving_grace(n);
         u.uhp -= n;
+        showdamage(n);
         if (u.uhp < 1)
             done_in_by(mtmp, DIED);
     }