post-processing merge should preserve "encoded date" metadata

[yoosefi]

• [ x] I've verified and I assure that I'm running youtube-dl 2016.05.10
• [x ] At least skimmed through README and most notably FAQ and BUGS sections
• [ x] Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• [x ] Feature request (request for a new functionality)

When merging/muxing youtube video components into a single file, the "Encoded date" / "Tagged date" is set to the year 1904, even though the source video and audio have proper encoding dates.

Applying --postprocessor-args "-map_metadata 0" via ffmpeg will copy it over, but this should be a default behavior.

Proposed solution: include -map_metadata 0 by default or use -metadata creation_time="DATE", where DATE is ISO-8601