Added validations for interactive version
Fixes #66 defaults preference to the best mp4 available when the format
isn't available

I also fixed some issues from my previous commit (the repeated question
and the bug in the interactive approach).
Fixes issue #68, edx-dl broke when no edx subtitles available

I added a validation and fixed the return type of the function to None.
Fixed œ type
Merge pull request #67 from iemejia/master

Fixes #66 defaults preference to the best mp4 available when the format … isn't available