Deferred testing / on-demand feature tests

[paulirish]

has.js makes you run a method when you want to test a feature. I believe this is for two reasons

1. feature tests can have a runtime cost. it's small, but it exists.
2. there are some tests that must be async (data URIs for example)

In another ticket we're talking about a pub/sub events setup for async events. But I wanted to focus on runtime cost.

What about using getters via Object.defineProperty to defer the initial run of the feature tests......

...

Oh i just realized this doesn't work at all for anything that you'd want a css-class for.

(Again, has.js has you opt-in to a css-class, which tackles that situation)

Well..

Discuss?

cc @pamelafox

[paulirish]

closing in honor of related tickets #589 and #622