Bot tweets opt out

Give #100DaysOfCode community members the option to opt out of receiving tweets from the bot.

They *could* just *block* the bot or *mute* it if they prefer but I think this will be a nice way to let users know that we're thinking of them and they can still follow the bot and see its posts in their stream so then be able to encourage other community members without getting tweeted at by the bot.

**opt out reply 'something' to stop the bot**

Something to that effect, this will have to go into a persistent datastore `LevelDB` is probably the best option.