Add ability to inject middleware so addons can become much smarter

[ndelangen]

Currently addons have access to the 'Manager App' and the 'Preview Area'.

Adding an ability to inject custom (express) middleware would open up even more opportunities for creative addon-creators.

These custom middlewares can provide storybook with new features by enabling additional data retrieval for addons.

Some data examples of data:

• unit tests
• package / dependencies version & updates
• package dependency graph
• git & other system integrations

I don't think it's all that hard to implement via config.js.

[thani-sh]

Hi @ndelangen
There's an open pull request for this feature #435. It does this using a middleware.js file. If we use the config.js file for middleware, it may add server side code to the javascript bundle it builds for the client.

[arunoda]

I think we can close this as we are working on this with #435