Adds support for YAML configuration files

[cmattoon]

I converted the JSON examples to YAML with yq and did some quick smoke testing with those files. Fixes #65

This file didn't work though (YAML or JSON):

$ ./bin/grafterm --cfg=./dashboard-examples/json/test-grid-fixed.json
error executing program: error on  widget grid position: widget grid position in a fixed grid should have an X position