Modified `docker export` to allow an --output flag

Copied code from CmdSave into CmdExport. This should work, not an expert in the API calls being made. But it does make more sense to have a consistent export/save flag.

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

checkpoint before edits on the export functions

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

Added an --output flag to docker export and created tests.

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

White space cleanup.

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

Docker-DCO-1.1-Signed-off-by: Joseph Kern <jkern@semafour.net> (github: jfrazelle)

checkpoint before edits on the export functions

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

White space cleanup.

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

Added text to reflect a new output option for the export command.

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

Whitespace clean up

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

Added man page documentation for the new --output flag in export

Signed-off-by: Joseph Kern <joseph.a.kern@gmail.com>

docs/sources/reference/commandline/cli.md

@@ -1068,14 +1068,22 @@ This will create a new Bash session in the container `ubuntu_bash`.
 
 ## export
 
-    Usage: docker export CONTAINER
+    Usage: docker export [OPTIONS] CONTAINER
 
-    Export the contents of a filesystem as a tar archive to STDOUT
+    Export the contents of a filesystem to a tar archive (streamed to STDOUT by default)
 
-For example:
+      -o, --output=""    Write to a file, instead of STDOUT
+
+      Produces a tarred repository to the standard output stream.
+
+   For example:
 
     $ sudo docker export red_panda > latest.tar
 
+   Or
+
+    $ sudo docker export --output="latest.tar" red_panda
+
 > **Note:**
 > `docker export` does not export the contents of volumes associated with the
 > container. If a volume is mounted on top of an existing directory in the