Any unknown commands should just be piped through and not use up an acknowledgement. This
new behaviour can be overridden via the new feature flag "unknownCommandsNeedAck", which will
restore the former behaviour causing even unknown commands to use up an "ok".
Also no line numbering or checksumming will happen. Shouldn't usually be of relevance for other
firmwares, but in case of any compatibility issues introduced by this also added a new feature
flag "sendChecksumWithNonGcode" that causes even unknown commands to be sent with a
checksum if necessary.