allow for index files to be resolved from the indexCache

pathCache creates one entry per `index.html` file, so the `index.html` files with the same path (e.g. as a result of multiple branches being served simultaneously) that are registered afterwards are disregarded, and saved in indexCache.

The issue is we are never looking anything up in indexCache when it is requested. It was impossible before this because there was no way to look up the required `app` and `branch` vars.

This PR adds that ability. As a result `HulaHoop.endpoints.index` can be called with two optional parameters to get the correct index file.

written by @jhudson8 and @patrickkettner


Does this not support platform? If not we should add platform to this. If so, then we should just use this. Index files are the only ones that are allowed to conflict in the namespace so providing such a selector option to the asset lookup methods will likely confuse users.