More formatting fixes

This pull request (Re)-fixes #9 by implementing almost all of the [Slack text formatting spec](https://api.slack.com/docs/formatting). When receiving messages we'll convert Slack's `<!everyone>`, `<#C1234>` and `<@U1234>` formatting into the equivalent text before passing it onto the rest of hubot.


Might be a nice place to use coffeescript's multi-line regex syntax here to make this easier to read

```coffeescript
txt.replace ///
  \< # Opening angle bracket
  ([\@\#\!]) # ID prefix
  (\w+) # Words
  (?:\|([^>]+))? # Uhh...something
  \> # Closing angle bracket
///g, (m, type, id, label) =>
  label
  # rest of function...
```