Let the user choose what output they want from benchmark

[johndoknjas]

Currently the benchmark function returns the "Nodes/second" output line after running the bench command. However, generally the "Nodes searched" line is of interest as well. So maybe add a parameter to benchmark allowing the user to specify what piece of info they want, and consider just returning the int value (rather than a string of the whole line).

On a separate note, consider making params a default Optional param equaling None. If it is None, just send the bench command without anything else to Stockfish.