Add TF1IE

I open this PR because I have some questions about this new IE, but first some info: TF1 uses the wat.tv player so TF1IE just extract the wat.tv url.

1. Which id should have the result: the one from TF1 or the one from wat.tv?
2. Which extractor should be set in the `extractor` field of the info_dict?
3. TF1IE should be in another file?

If the answer to the first two questions is that TF1IE should overwrite some of the fields of the result, I have in mind to make TF1IE a subclass of WatIE, so that in can directly call the `extract` method.