libjiyuubot: require commands to be NORMAL
libjiyuubot: require commands to be NORMAL

This commit is a short one: if a command handler is activated with a
message that isn't MessageType.NORMAL, it returns.

Bob131/JiyuuBot-ng,Bob131/JiyuuBot,Bob131/JiyuuBot-ng,Bob131/JiyuuBot-ng