Add CSS transition support

I agree it sounds weird in the context of the component class, but it's meant to be specified/overridden on the component in the template, which reads well (IMO):

```handlebars
{{#lio-content disableTransitions=true}}Your mom.{{/lio-content}}
```

In that context, do you still think it should change? We also need to figure out a convention for class properties that are meant to be specified as handlebars attributes.