Keep Uploaded Date/Time after merging

[ghost]

eg. when downloading a file using -f bestvideo+bestaudio the resulting file has current date/time instead of when the original video was uploaded.

[dyn888]

@jaimeMF This could be closed since it was implemented.
@dfortner Server date will be used by default now, use --no-mtime to turn it off. Keep in mind in case of YouTube file date will change every time they move or restore some file, which happens a lot. It's better to use %(upload_date)s in the output filename template (use double percent %%(upload_date)s in Dos/Windows batch files).

Example:
-o "%(title)s [%(upload_date)s][%(id)s].%(ext)s"