Add mpegts support

[SpacyNG]

I've added support in #1682 for motion to be able to handle Mpeg Transport Streams containers.
I'd like to hear about your opinions and maybe test results.
If your webcam supports a mpegts, set netcam_url to something like http://192.168.1.2/stream.ts and set or add to netcam_params format=mpegts
For old configurations that use mjpeg, you may either omit the format parameter (like before) or explicitly set it to format=mjpeg.
I've tried to keep it fully backwards compatible.

In theory this parameter now allows any demuxer that ffmpeg/libav provides, but I only tested it for mjpeg and mpegts for now.

If this change is welcome, I'll update the documentation and finish the PR.

Let me know what you think.