Add support for imageio for accessing the webcam

[krumnack]

Imageio now provides an alternative way to access the webcam (based on ffmpeg)
(see here) which seems to work in some cases where opencv/gstreamer does not work.

It may be worth adding an option to choose which library (cv2/imageio) to use for accessing the webcam.

[shoeffner]

Thank you for pointing out imageio, I wasn't aware of it yet.

Looking back at #21, it definitely makes sense to provide multiple drivers, so I will take a look at integrating imageio as an alternative.