add config to enable MJPG encoded stream

[machinekoder]

This patch allows forcing MJPG steam support of the camera.

[furushchev]

Thank you for your contribution.
If I understand correctly, when enable_mjpg is set to true and a camera device supports mjpg, image encoded as mjpg is published directly without decoding to bgr8 (as usual image), right?
If yes, I'm wondering we should use sensor_msgs/CompressedImage message instead of sensor_msgs/Image.
Another question is some image manipulation like cv::flip is supported on mjpg image?

[machinekoder]

As far as I understand, this only forces the USB camera to use MJPEG encoding to transport the image from the camera to the PC, which adds additional CPU load to the PC for decoding and of course decreases the stream quality. Depending on the camera this may improve the frame rate of the capture stream. The captured image encoding in OpenCV is not affected by this change.

[machinekoder]

@furushchev @awesomebytes Rebased on master. On a new camera I have here, enabling MJPG stream is necessary to unlock all resolutions.

[awesomebytes]

Looks good to me, what do you think @furushchev

[awesomebytes]

That said, @machinekoder would you mind adding to the documentation in the README this new option?

[machinekoder]

@awesomebytes done