fix(api): Selector strategies must have context.

At some point I said I was going to incorporate the new api into my
Astrolabe tutorial, and I did today. I was upset to find that these
new functions don't work. Fortunately it was a simple fix.

I can now attest that these functions are working in a real life app.

This definitely raises the concern that without end to end tests,
new changes like this aren't going to be tested until they're put
into a real application. Expect a pull request for some kind of an
"example app" directory, complete with end to end tests for each
kind of element and selector.