Add `--load-info` option

This my proposal for #972.

How it works:
1. You write the info.json with `youtube-dl {URL} --write-info-json --skip-download`
2. You run `youtube-dl --load-info 'path.to.info.json'`.

Internally it just calls  `process_ie_result`, so all the options will work (post-processors, format selection ...).

Some other things:
* Philipp  suggested storing the cookies in the info.json. I haven't look into implementing it, but using the `--cookies` option will also work.
* I'm not really sure about the option name, any suggestion?