(HI-281) Add YAML output format to CLI

Just add YAML output to HI-129. I use hiera with other tools like
Salt (custom 'backend') and yes, i can directly parse from JSON.
But YAML is more "elegant" than JSON and I can directly use
safe_load with pyyaml.
Test: ```hiera -f yaml -c /etc/hiera.yaml key```
Merge pull request #198 from ahmet2mir/master

(HI-281) Add YAML output format to CLI