kill ffmpeg process on finish up if enabled

[yvd]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	fixes #issuenum
Related issues/PRs	#issuenum
License	MIT

What's in this PR?

Added an option to kill the ffmpeg process in the finish_up phase. Useful when using an input source as camera

Why?

Need to deallocate input source (camera) programatically once the stream is finished, so that it can be used again.

Example Usage

video = ffmpeg_streaming.input("/dev/video0", capture=True)
hls = video.hls(Formats.h264(), kill_process_on_exit=True)

// Now we can do
hls.finish_up() // will stop the ffmpeg process and free the attached camera


#### To Do

- [ ] Create tests