Adding ability to pass in a className

This is an enhancement for issue #10. I've added a test and everything is green on my machine.

The biggest change here is that messages are now objects, rather than simple strings. This allows us to attach extra information to them. As an example:

```
{{#flashMessage}}
    <span {{bind-attr class=":message message.type"}}>{{message.text}}</span>
{{/flashMessage}}
```

Let me know if you want anything done differently. Thanks for the great library! :heart_eyes: