general --mirror option

[chrysn]

• I've verified and I assure that I'm working with youtube-dl git master (b0582fc) (to be precise: i read its README, because even if i ran the code, i wouldn't have found the feature if it's not in the man page)
• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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Description of your issue, suggested solution and other information

when running youtube-dl, i usually want to make sure to have as much (meta)data available locally as i would have if i used the video page online; thus, my typical command line arguments are --all-subs --write-description --write-info-json --write-annotations --write-all-thumbnails. (no --all-formats for that is not likely to contain any information that can't to a reasonable approximation be obtained from the best format; under the same argument, --write-thumbnail might be the better choice here.)

even if i were sure that i'm not missing a feature (i did miss --all-subs for some time), such an invocation is not future-proof against new metadata extraction features (eg. if at some point there'll be a --write-user-comments). would you consider adding a --mirror or similar flag that enables a sane set of "i want to be able to run a mirror of this if the site goes down" flags like there is for wget?