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>
Had --o instead of -o for the short version of the flag. Now matches help text.

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

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