Add support for caching files already on disk

[CraigMason]

We use PINCache (previously TMCache) as part of our caching layer, and we use PINDiskCache as an offline store for image assets and other metadata.

A great feature would be to support the caching of existing files provided by an NSURL.

Use case for us is NSURLSession background session. We download > 2000 files using a background session, which provides a callback with a temporary file url. We have implemented a separate system to store offline assets, as it includes videos, ZIP files, PDFs, but would love to have it all wrapped up as part of PINDiskCache.

[biogerm]

Hi @CraigMason ! How far did you go with development?