Added FFmpeg recorder support

Screen Recorder tab now has a checkBox: Run CLI afterwards so that you
may execute your favourite Command Line Interface (e.g. x264.exe,
ffmpeg.exe) to encode what has been recorded by ShareX. You may choose
Video for Windows (VfW), FFmpeg or Animated Gif to record your screen.
VfW will allow you to create uncompressed AVI streams or compressed
streams using x264VfW etcc; FFmpeg will allow you to create MPEG4
streams - this is handy if you do not have Administrator access to
install x264VfW to compress your streams. Run CLI afterwards checkBox is
disabled for Animated GIF until we actually see a CLI that could take
GIF and output to a movie file.