Add support for preview-decorators.

[arunoda]

With this, we could set a decorator for preview via an addon.
This is pretty useful for Storybook Designer where we could add more tools to the preview area.

Currently, it's only possible to set only one preview decorator for the whole app.

[arunoda]

@mnmtanish Check this out.
I also did this: storybook-eol/storybook-addons@c4d256c

[thani-sh]

We can give this feature without introducing another api function. In React Native Storybook it's done like this. To set a custom preview area, we just have to write an addon and call addons.setPreview in there.

For React Storybook, we can simply move the iframe code to an addon storybook-addon-iframe-preview and include it in addons.js file by default. If anyone wants to add a custom preview area (designer?) they can simply fork the iframe preview addon and change it as they wish.

I strongly suggest we remove the Preview decorator API methods from storybook-addons repo ASAP and use the existing one.

[arunoda]

Okay. I'll revert.
No need to move iframe code into another project.
We can work with the old API.

[thani-sh]

I'm just saying that it'll be really easy to fork and extend it it's in another project

[arunoda]

No need to fork and extend. We don't want to replace the preview. Just need to decorate it.
So, I we don't need this code at all.

[thani-sh]

Let's agree to disagree then