Add initial implementation of a v4l2 capture plugin

This patch adds a plugin to capture video data from v4l2 devices under linux.
v4l2 plugin properties
added ability to choose device, pixel format, resolution
and frame rate
Merge branch 'v4l2-input' of https://github.com/bazukas/obs-studio into bazukas-v4l2-input
Merge branch 'bazukas-v4l2-input'