Increase outflanking weight to 12

Increase outflanking weight to 12

Give more incentive to king activity in the endgame by increasing the weight
of the "outflanking" variable from 8 to 12 in the function evaluate_initiative().

Finished yellow after 133102 games at STC:

LLR: -3.07 (-2.94,2.94) [0.00,4.00]
Total: 133102 W: 29535 L: 29179 D: 74388
http://tests.stockfishchess.org/tests/view/5b2b63fe0ebc5902b2e54475

Passed LTC:

LLR: 2.95 (-2.94,2.94) [0.00,4.00]
Total: 28027 W: 4918 L: 4672 D: 18437
http://tests.stockfishchess.org/tests/view/5b2ba39e0ebc5902b2e54a64

Closes https://github.com/official-stockfish/Stockfish/pull/1657

Bench: 4721753