Add json stdin

Work on stdin:
  - [x] Update docs
  - [x] Modify MapData to map[string]interface[] from map[interface{}]interface to x  make using JSON easier
  - [x] Add JsonData methods and Map() implementation
  - [x] Rename inject param from `yaml_input` to `dest_file` to make it obvious the param is the destination data file.
  - [x] Use JsonData when stdin is passed in to the command