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>