ability to pass in a maxWidthClass class name binding

I'd like the ability to have the input look different when it reaches max width. Maybe something like:

```
{{input autoresize=true maxWidthClass="lol cats" max-width=300}}

## initial rendering

## when the width hits 300px

```

If this sounds OK to you, I'll make a PR. @tim-evans

also, perhaps an action that can be fired when we reach max width. I feel more iffy about this though.