Enable kill and renice running ffmpeg process

This change adds a FffmpegCommand#kill method to send a signal to the command's ffmpeg process. It defaults to 'SIGKILL' but you can specify a different signal name (eg. 'SIGSTOP' or 'SIGCONT' to pause/resume) as an argument.

It also enables FfmpegCommand#renice to apply a new niceness value to ffmpeg if it is running.