Add flag to read JSON from file

[emersion]

A new -i flag is introduced to allow users to pretty-print a JSON dump. Users can now share JSON dumps, and optionally post-process them, e.g. this can be used to only print the connectors:

drm_info -j | jq '. | map_values(. | {driver,device,connectors,encoders:[],crtcs:[],planes:[]})' | drm_info -i