Add Spam Filtering

[andrewlalis]

Add the ability for the bot to detect and handle spam by users. This should be able to detect when a user does any of the following:

• Sending many messages in a channel.
• Sending messages in many channels in a short time.

All messages determined to be spam should be removed, and the user should be muted. This means that the implementation will probably have to cache messages as they come in, so that it can remember all spam messages in the short term.