Create a this.cacheRoot(), like this.sourceRoot() or this.destinationRoot()

The idea is to have a function that gives the root cache directory used as root for `.remote()`

Indeed, I'm using `.remote()`, and I also fetch repositories from other place than github, and I also need a place to launch some skeleton process, the best place for that is a cache directory somewhere.

So why not using the cache already set by `remote()`, and let it available for everyone ?