Don't force @providesModule on the masses

Right now we always treat @providesModule directives as more important
than anything else, with no escape hatch. This change will make it
opt-in, which I think makes sense given that React build process is the
consumer.

See https://github.com/facebook/react/issues/1145
Add tests, fixup existing test flow

This makes the context instantiable since you can only set options once.
I didn't change *all* tests, only ones using waitForHelpers. Others
still use the global shared contexts.
Merge pull request #66 from zpao/provides-module-option

Make @providesModule handling opt-in.