Write a downloader for m3u8 manifests

[jaimeMF]

While trying to solve #986 I found that HD videos can be downloaded using the data contained in a m3u8 manifest, a quick hack can be used to download all the parts of the video
But I think it will be more usefult to write a downloader for m3u8 manifests, so all sites that use this format can be easily added and the user get a unique file with the complete video. For that 2 steps need to be done:

• Write the downloader: once we have a manifest is easy to download each of the parts.
• Write a postprocessor that joins the video parts, we can use ffmpeg for that: example script

If you find that is better to just use the method I have already posted, I can clean that and commit it.

[jaimeMF]

I've think more about it and it's better to just support multpart videos in general in YoutubeDL instead of writing a new downloader just for m3u8

[jaimeMF]

Reopened after discussion in #995.
Once it's merged, write a downloader that calls the postprocessor after downloading the files.
Remember to add support for encrypted fragments (youtube uses them).