As the "LG Time Machine" (a (not so) smart TV) has a limitation for video dimensions (as for codecs), I take to implement an extra parameter `--pp-params` where we can send extra parameterization for the video converter (post-processor).
Example:
```
$ youtube-dl --recode-video=xvid --pp-params='-s 720x480' -c https://www.youtube.com/watch?v=BE7Qoe2ZiXE
```
That works fine on a 4yo LG Time Machine.
Closes#5733
Without the '--keep-video' option the two files would be downloaded again and even using the option, ffmpeg would be run again, which for some videos can take a long time.
We use a temporary file with ffmpeg so that the final file only exists if it success
* Remove the 'songtitle' field, 'title' can be used instead.
* Remove newlines in the help text, for consistency with other options.
* Add 'from __future__ import unicode_literals'.
* Call '__init__' from the parent class.
* Add test for the format_to_regex method