Add Marionette.js support

[avasuro]

Describe the solution you'd like
I'd like to see Marionette.js support, as I see Storybook to be very component focused, and Marionette being a very component focus framework - these tools would compliment each other very well.

Are you able to assist bring the feature to reality?
yes, I can.

[stale]

Hi everyone! Seems like there hasn't been much going on in this issue lately. If there are still questions, comments, or bugs, please feel free to continue the discussion. Unfortunately, we don't have time to get to every issue. We are always open to contributions so please send us a pull request if you would like to help. Inactive issues will be closed after 30 days. Thanks!

[shilman]

Great Caesar's ghost!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.0-alpha.0 containing PR #7981 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.