Allow users to specify ID:camera mapping #74

Feature request: somehow save to config how we assign camera id to each camera. Allow user to specify which camera should be 0 which 1 ..etc. based on camera serial number or similar.

For me it currently assigns the main camera as 1 and secondary as 0, for consistency i would like to have the main camera as 0.

Originally posted by @arturoptophys in #40