Add string option /pieceNickNames

Add string option /pieceNickNames

This (volatile) option allows definition of alternate letters for the
pieces, which will be recognized next to the standard symbols as given
in the /pieceToCharTable on input. (Output always uses the standard.)
This can be useful for reading PGN from another language, or
non-compliant FENs (e.g. using B and N for E and H in Xiangqi).
  The format of the argument is the same as for /pieceToCharTable.
Because the nickNames are consulted before the standard table, it is
possible to redefine a standard name as a nick for another piece.