Enhancement: Make snapshot directory structure of mount command custimizable

We've added the possibility to customize the snapshot directory structure of
the mount command using templates passed to `--snapshot-template`. The
formatting of the time for a snapshot is now controlled using `--time-template`
and supports subdirectories to for example group snapshots by year. Please
refer to the help output of the `mount` command for further details.

Characters in tag names which are not allowed in a filename are replaced by
underscores `_`. For example a tag `foo/bar` will result in a directory name
of `foo_bar`.

https://github.com/restic/restic/issues/2907
https://github.com/restic/restic/pull/2913
https://github.com/restic/restic/pull/3691
