content: protocol in links

I'm running into trouble with Chaplin's built-in link click handler in a Trigger.io app. Trigger uses the `content:` protocol for their pages, which doesn't get along well with this code:

```coffeescript
# layout.coffee
return if isAnchor and (
  # ...
  el.protocol not in ['http:', 'https:', 'file:']
)
```

Can we either add `content:` to the allowed list, or make it configurable, maybe with a setting passed to `Chaplin.Layout`?