Add experimental contempt factor

This is very crude and very basic: simply in case
of a draw for repetition or 50 moves rule return
a negative score instead of zero according to the
contempt factor (in centipawns). If contempt is
positive engine will try to avoid draws (to use
with weaker opponents), if negative engine will
try to draw. If zero (default) there are no changes.

No functional change.

src/ucioption.cpp

@@ -64,6 +64,7 @@ void init(OptionsMap& o) {
   o["Search Log Filename"]         = Option("SearchLog.txt");
   o["Book File"]                   = Option("book.bin");
   o["Best Book Move"]              = Option(false);
+  o["Contempt Factor"]             = Option(0, -50,  50, on_eval);
   o["Mobility (Middle Game)"]      = Option(100, 0, 200, on_eval);
   o["Mobility (Endgame)"]          = Option(100, 0, 200, on_eval);
   o["Passed Pawns (Middle Game)"]  = Option(100, 0, 200, on_eval);