(HI-129) Add ability to specify output format

This adds the ability to specify the output format from the hiera
command line. Supported formats are "ruby" (the default) and "json".
(HI-129) Add doc stating supported formats
Merge branch 'pr-172'

* pr-172:
  (HI-129) Add doc stating supported formats
  (HI-129) Add ability to specify output format