Implemented general go command

[jaxony]

Thanks for the UCI library @imor! Years down the track and it's still finding use 👍

With this PR, the user can specify exactly how their go command looks like with a new Engine.goCommand() implementation.

For example, to get stockfish to execute go movetime 5000 searchmoves e2e4 d7d5, the user can use Engine.goCommand(obj, callback) like so:

engine.goCommand({movetime: 5000, searchmoves: 'e2e4 d7d5'}, infoHandler);