Make pagesInit return a Promise with the Elm app instance

[icidasset]

Hi again 👋

I made this change in a fork for our project so we could set up analytics and use ports.
Is this of any use to you? I can make changes if need be. Allows you to do:

const { Elm } = require("./src/Main.elm")
const initializePages = require("elm-pages")

initializePages({
  mainElmModule: Elm.Main
}).then(app => {
  // app.ports.somePort.subscribe(...)
})

[dillonkearns]

Fantastic, this is excellent! Tried it out and it works very smoothly. Thanks so much @icidasset! 🙏

[dillonkearns]

@all-contributors please add @icidasset for code.

[allcontributors]

@dillonkearns

I've put up a pull request to add @icidasset! 🎉

[dillonkearns]

Released! https://github.com/dillonkearns/elm-pages/blob/master/CHANGELOG-NPM.md#122---2020-01-27