Upgrade ffish.js to 0.4.4 and add validate_fen

Upgrade ffish.js to 0.4.4 and add validate_fen

* Upgraded ffish.js to 0.4.4
+ validate_fen(const std::string& fen, const Variant* v) (Closes #87)
+ ffish.validate_fen(fen)
+ ffish.validate_fen(fen, uciVariant)
+ ffish.startingFen(uciVariant)
+ board.pocket(turn)

Refactoring
+ replaced bool first = true by save_pop_back(string& s)
+ replaced string = " " + s; by string += " "; string += s;

updated README.md
+ simplified memory specification

* Added C++17 specification for building ffish.js
+ updated ffish.js to 0.4.4

* Added more checks to validate_fen()
+ FEN_TOUCHING_KINGS
+ replaced FEN_INVALID_CHECK_COUNTER by FEN_INVALID_NUMBER_OF_KINGS
+ combined FEN_FILE_SIZE_MISMATCH and FEN_INVALID_NUMBER_OF_RANKS into
FEN_INVALID_BOARD_GEOMETRY
+ added more checks for FEN_INVALID_CASTLING_INFO

added struct CharSquare
added class CharBoard (for geometry checks)
refactored different checks into separate functions