Basic config object allowing disk path to be configured

[eimermusic]

This commit moves the disk cache root path (and domain identifier) into a config object you can swap out in your app delegate.

// in AppDelegate.swift
HanekeGlobals.config = Config(
    domain: "images",
    cacheRootPath: NSSearchPathForDirectoriesInDomains(FileManager.SearchPathDirectory.documentDirectory, FileManager.SearchPathDomainMask.userDomainMask, true)[0]
)

I have a use-case where the cacheable images and video clips are ephemeral and one purpose of the iOS app is to allow local persistence of the data. I also saw that other users has posed questions in this direction. E.g. #423

I can also see my app wanting to expose these images to the Files app, which is why the domain identifier came along for the ride.

I am so impressed by the simplicity and performance of Haneke. I really wanted to keep getting the benefits of that while still providing persistent storage.

Thoughts?

[eimermusic]

Totally forgot about the tests yesterday. I hope this makes travis happy.