Introduce named subject for rendered component

[boardfish]

@andapopovici and @pythonandchips suggested they'd like to see this feature. Right now, the rendered component is an anonymous subject.

Potentially regardless of whether folks prefer one-liner specs or not, a named subject would be useful for the odd case where you do see yourself making a full it .. do block, but optimally, I'd expect the vast majority of component specs to be able to take this one-liner style.