Non interactive (WIP)

Hi, George and others.

I just started rewriting the script to be less interactive, since I don't want to be bothered answering the same questions over and over (say, the format of the videos and so on).

Please, take a look at these changes and feel free to give your comments.

I could, of course, just have pushed these commits to the master branch, but I decided that it would be wiser to *not* merge this pull request myself and to just get some feedback on what a proper future for edx-dl we would like to have.

Regards,

Rogério Brito.