Generate asset index file

Add support for generating an asset index file, mapping original paths to versioned paths.

If the `indexPath` setting is configured then the asset index is written to that path (with checksum and fingerprinted files for the index also added).

The default is to write a json / javascript object. The writing of the index file is also completely overridable by specifying the `indexWriter`, a function of `Map[String, String] => String`.

See: https://github.com/playframework/playframework/issues/3435

And has been tried with requirejs plugins and requirejs load function overrides. See: https://github.com/pvlugter/play-require-sample/tree/assets-index and https://github.com/pvlugter/play-require-sample/tree/override-load