Add a `hidden: true` option

[emilymdubois]

This library enforces a paradigm that is reinforced the more it's used in a project: prop permutations go in test_cases files, and test files iterate through the test cases and make assertions.

However, there are certain components (e.g., higher-order components) that I may want to exclude from the test kitchen component without introducing a different testing structure paradigm. One idea would be to introduce a hidden: true option into the test case API that filters out that case from the final component.

[davidtheclark]

@emilymdubois Ah, I realize I forgot to document this: the convention @danswick and I were trying out was to export a different object with test cases that you don't want to be displayed. Anything in testCases will be displayed — but you can export whatever else you want and import them into your test file. So you could export a unrederedTestCases object and use its contents in your unit tests, but React Test Kitchen won't try to render it. @danswick does that sound right?

[danswick]

Yes, that jives with how I remember it, @davidtheclark!

There's a superfulous noRenderCases export here from when we were discussing this.

And the build-index.js script that ultimately drives the demo page checks each item in the index, but only checks them for exported testCases and nothing else.