Manually add subtitles to download videos

[bytedream]

Until now, Crunchyroll always provided videos with subtitles burnt-in. But it seems that they starting to shutdown this kind of videos, at least for some languages. Italian for example has no hardsubbed videos for some series (#81 (comment)). To prevent such errors as in #81, this PR removes the logic to download the pre hardsubbed videos and burns in the subtitles itself via ffmpeg.

This includes some major changes:

• ffmpeg is now required for download too. Adding subtitles to a video cannot be done without it.
• The default file extension will change from .ts to .mp4. Mp4 is one of the few container formats which supports subtitles to be added as a separate stream which can be turned on and off (and not directly be burnt into the video as it's the only choice for most other formats). This has the advantage over .ts that the whole video doesn't need to be re-encoded which takes some time, especially with higher quality videos. All other formats will burn the subtitles into the video which consumes much time and cpu. It's now highly recommended to use mp4 as format for download. The subtitles in a mp4 file will have the ffmpeg forced flag/disposition which should show them in the most video players automatically but you might activate them manually in players which does not recognize this flag.
• Piping to stdout (with the -o - flag) ([Request] Add option to pipe download to stdout #69) isn't so fast anymore. In the past when the actual video download started, the piping process started immediately. But now this isn't possible anymore b/c ffmpeg requires the output file to be seekable in order to add subtitles (at least for mp4, which is the format which will be piped). To have this guaranteed when piping, download will create a temporary file which gets piped to stdout if the video content was fully written to it. This makes piping less powerful than before.