Updated ffish.js to 0.4.2 (Closes #185)

Updated ffish.js to 0.4.2 (Closes #185)
+ board.moveStack()
+ board.pushSan(sanMove, notation)
+ board.pushMoves(uciMoves)
+ board.pushSanMoves(sanMoves)
+ board.pushSanMoves(sanMoves, notation)
+ ffish.readGamePGN(pgn)
+ game.headerKeys()
+ game.headers(key)
+ game.mainlineMoves()
+ ffish.variants()
+ ffish.loadVariantConfig()

Added parse_istream()
+ to allow reading a variant configuration file via a stringstream

Added NO_THREADS #define
+ to disable threads usage for build

Added custom variants info to README.md

Changed ordering for ffish.loadVariantsConfig() in test.js