Restore whole folder to sdtout as tar

With this change it is possible to dump a folder to stdout as a tar. The

It can be used just like the normal dump command:

`./restic dump fa97e6e1 "/data/test/" > test.tar`

Where `/data/test/` is a a folder instead of a file.

doc/050_restore.rst

@@ -124,3 +124,13 @@ e.g.:
 .. code-block:: console
 
     $ restic -r /srv/restic-repo dump --path /production.sql latest production.sql | mysql
+
+It is also possible to ``dump`` the contents of a whole folder structure to
+stdout. To retain the information about the files and folders Restic will
+output the contents in the tar format:
+
+.. code-block:: console
+
+    $ restic -r /srv/restic-repo dump /home/other/work latest > restore.tar
+
+