Allow overriding/adding ffmpeg args.

[bbaldino]

As mentioned here, it could be useful to override some/pass additional ffmpeg args. Getting into a situation where we have to 'merge' args would be a bit tricky, so maybe we want to allow them to be overridden entirely, or only allow overwriting certain arguments.

[netaskd]

It would be grate to have the support of custom flags for ffmpeg. It opens a way for creating multi-instance jibri on the same host, because for now we are bound to hard-coded interface "-i hw:0,1,0"