New backend proposal: ProxyFS

[billiegoose]

I just had this idea as a way to implement the file-system watcher described in #163

We could have a backend that wrapped another file system and provided hooks so you could intercept any of the method calls and add additional logic. In this scenario, I would simply intercept write, mkdir, rmdir, rename, and unlink to easily create my own "file system watcher".

ProxyFS is meant to allude to JS Proxy objects, but other names I came up with first included HookFS, WatchFS, SpyFS, or InterceptorFS.

[james-pre]

Please use zen-fs/core#9