More control over decorators for addon developers

[thani-sh]

Allow addon developers more control over decorators. Sometimes they may need to apply decorators to only some parts exported by the addon or ignore them completely.
Example usecase: storybook-eol/react-storybook-addon-info#40

That being said, this makes addons behaviour more ambiguous and harder to debug.

[arunoda]

I hope we already discussed about this.

[ndelangen]

Addon info's problem is it should actually render in panel, #1147

I don't see what steps we should take, what control do they need?
#1209