How to add a new downloader method

[jaimeMF]

I've written a downloader for f4m manifests (the code is in this repo: https://github.com/jaimeMF/f4m-dl), I want to integrate this in the youtube-dl code. This is my proposal:

1. Move all the download code to a new downloader module.
2. Use FileDownloader as the base class and move the HTTP file downloading code to a new method.
3. Write a class that would be used by YoutubeDL that given an info dictionary decides which method it should call.
4. Allow to specify which downloader to use use in the info_dict, like: 'downloader': F4M_DOWNLOADER (where F4M_DOWNLOADER is a constant from the downloader module)

Instead of creating a new module and new classes it may be better to just add the f4m download process tas a new method, as with rtmp. But I think that all the flv handling code shouldn't be in FileDownloader.py, that's why I prefer the other method, although this is simpler.

There's also more reasons why I think is better to split the code in different classes:

• When using the f4m downloader multiple files are downloaded, which will produce different progress_hook calls, but the listener may only be interested in the final file progress, not the individual file.
• Sightly related to the previous point, currently there's a progress viewer for each fragment, but trying to use only one for the final video (some videos are divided in more that 100 parts) is quite hard if it's a method from the same class.