Implement a plugin architecture

[sillygwailo]

Largely done, just testing it out with the dev version deployed to Heroku. Based on the approach at https://github.com/chris--young/Slacker though it doesn't have collision detection. In the future, I hope to have the following plugin types:

Plugin types and examples:

• message - takes a Slack message object, though I can't think of what to do with it
• tweet - takes the tweet from the Twitter stream and looks at the links, tries to add value based on the metadata contained within
• file - takes a file upload (images)
• filter - remove formatting, add smart quotes, convert emoji short codes to real emojis, etc. (these are already implemented). It just receives the text property of the message object from the Slack user making posts to Twitter and runs replacements. The 140-character check is done after all the replacements.