[Feature]: Add a file watcher

[KonnorRogers]

not all files should need to be imported by your bundler. Right now to get the most performant bundle you need two entrypoints.

• app.js
• assets.js

This can get confusing as to "when should I import assets in the app bundle or the asset bundle"

It's also particularly annoying if your bundler doesn't accept glob imports.

As a result, AssetMapper should accept a Rake FileList of files to watch.

AssetMapper.new.configure |config|
   config.file_watcher.files = FileList.new("app/assets")
   config.file_watcher.manifest_file = "public/assets/asset-mapper-manifest.json"
end

importantly the file list should probably exclude CSS and JS assets.

[KonnorRogers]

Im still undecided on this. It adds a lot of complexity, it goes against the mantra of "let your bundler handle your assets", and it introduces 2 ways to handle things.

However, its also reasonable that people may not be using a bundler at all and all they want from AssetMapper is asset fingerprinting.

EDIT: I think I just convinced myself this is needed. It wont do asset compression, minification, etc. Just a simple file hasher.

[KonnorRogers]

Not sure why Sprockets went away from it, but Guard seems like an easy way to handle this.

https://github.com/stevehodgkiss/guard-sprockets2