Implemented option to watch other non-elm files and auto-reload on changes

[Levyks]

With this change, you can use the --watch (or -w) option to watch any directory or file of your choice, and the browser will reload in response to any changes on them

This is really useful for when you're editing a CSS file, with it, you're able to see your changes in the browser without having to reload the page

Usage example:

elm-live src/Main.elm --dir=./public --watch public/* -- --output=public/build/main.js

You can watch multiple directories/files by separating them with commas:

elm-live src/Main.elm --dir=./public --watch public/*,someotherdir/* -- --output=public/build/main.js

[netlify]

👷 Deploy request for elm-live pending review.
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🔨 Explore the source changes: b8a3a70