Updated an option to indicate which team (libav or ffmpeg developers) supplied ffmpeg, added ability to pass extra options to ffmpeg

There are two flavors of ffmpeg that one may need to work with. One is the kind supplied by the libav team (it is what you get on ubuntu when you install the packaged ffmpeg). The other is supplied by the ffmpeg team. These two versions have slight variations in their command line syntax. For example, the libav version accepts WIDTHxHEIGHTxBITs dimension, while ffmpeg developer's version expects only WIDTHxHEIGHT. Also the latter does not recognize the -g option.

Sometimes it is also required to pass extra parameters to ffmpeg.

This pull request addresses both items. It adds an option for passing which flavor of ffmpeg is used (:provider option). And it accepts an array of extra ffmpeg parameters which are passed to ffmpeg when it is launched (:extra option).
This is a public API breaking change. Previously ffmpeg was the default one. If you really want to make this change you should change occurences of ffmpeg in `Capturing video` section too.