Support for diffing kubernetes configmap content

Right now, `dyff` treats configmaps like every other resource, which means that if there's a one-line difference in a hundred-line file, `dyff` will print all 100 lines of both old and new content.

It'd be nice if `dyff` detected that a resource was a configmap and diffed the *content* instead of treating values in the configmap as atomic. (I hope that makes sense.)