* Adding a callback_command_not_found to allow plugins to "catch" any commands that were not processed. The goal is to allow a plugin to receive any message not already processed so that it may implement additional pattern matching or other functionality to process the message.
* Adding tests for command_not_found callback
* Rough draft for command_not_found core plugin
* Moving command-not-found functionality to core plugin
* Code style fixup
* Removing old rough draft using command_not_found callback
* Removing duplicate .replace for prefix test in cnf_plugin
* Updating plugin interface to use self.bot_config instead of _bot.
Updating command prefix check to use string instead of only character.
* Updating to use decorator parameter
* Refactoring catch_unprocessed flag set to more graceful .get