Original issue title: Add WebDAV server as alternative for fuse

Important: This is not about an WebDAV Backend.

I think the idea of using fuse to examine the backuped snapshots is great, but as some OS do not support fuse and somebody might not have enough rights to use fuse, it would be great to have an alternative.
My idea was to add a command to restic that instead of mounting the snapshots via fuse opens a local WebDAV server. As all major OS have a options to mount a WebDAV server, everybody would be able to mount the snapshots.

As I guess that it is easy to find a WebDAV serverlib for Go and you would use the same directory structure the fuse mount, it should not be too complicated to add this feature.