Require a root when initializing assets

[timriley]

Assets now requires a root, e.g.

assets = Hanami::Assets.new(config: Hanami::Assets::Config.new, root: "/some/path")

This makes it easier to use separate assets objects for different sets of compiled assets at different paths.

Along with this, make the default config more useful by providing a default value of "assets.json" for the manifest_path setting.