Added prompted command to make the bot respond whenever its name is mentioned

[bolecki]

Hello! I added a new command that will cause the bot to respond every time a message contains its name. This will override the default behavior of doubling the replyrate.

Use !prompted on or !prompted off to toggle this feature. You can use this with !replyrate 0 to make the bot only respond when its name is mentioned as well.

Updated from PR #29

Thanks!