Add a second level of follow-up moves

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 6438 W: 1229 L: 1077 D: 4132

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 4000 W: 605 L: 473 D: 2922

bench: 7378965

Resolves #636

src/search.h

@@ -28,6 +28,9 @@
 #include "position.h"
 #include "types.h"
 
+template<typename T, bool CM> struct Stats;
+typedef Stats<Value, true> CounterMoveStats;
+
 namespace Search {
 
 /// Stack struct keeps track of the information we need to remember from nodes
@@ -43,6 +46,7 @@ struct Stack {
   Value staticEval;
   bool skipEarlyPruning;
   int moveCount;
+  CounterMoveStats* counterMoves;
 };
 
 /// RootMove struct is used for moves at the root of the tree. For each root move