Proposal: allow runApp() to accept object with engine

[wch]

Currently, when you call runApp() and pass in some files, you do this:

runApp(el, "viewer", { startFiles: files }, "r")

Where files has type Array<FileContent>.

But I think it makes as much sense for the object that's passed to runApp to contain information about which engine to use. Instead of just an array of files, it would also provide info about how to run the app. So the structure would be something like:

type AppSpec = {
  engine: AppEngine,
  files: Array<FileContent>
}

// This could be loaded from JSON
const app = { engine: "r", files = [ ... ] };

runApp(el, "viewer", { appSpec: app })

Unlike some of the other options engine seems inherent to the app; viewerHeight, for example, seems incidental to the app.