Explore adding Stackbit to an existing application

[seancdavis]

Right now we look for a package.json and consider the project to be an existing project. The script then asks whether you want to add Stackbit to the site, and if yes, then outputs the URL to the integration guide.

Here we want to explore what it would look like to actually do that work. What would we add to the project?

Should it be the same command, or should the create-stackbit-app command output a message to use a different script, which may or may not be contained within this project? (Probably easier if we keep them together.)