Add "recompile on change" functionality to the compiler

[passiomatic]

I think a good example is the elm-watch tool, which recompiles whenever your Elm files change and reloads the compiled JS in the browser.

This is described in great details here: https://lydell.github.io/elm-watch/what-elm-watch-is/. Anyway, essentially it has:

• Hot reloading. elm-watch tries to reimagine the level of quality of hot reloading. The goal is to never leave you wondering if it worked at all.
• Browser UI. elm-watch always shows you the latest status in the browser. Switching to --debug mode – or even --optimize mode – is only a click away.
• Includes an optional server that lets you get going with a Browser.application. This is in beta but spares me to have a separate server to serve assets (images, CSS, etc.)

And more, but I think this could be a great start!