text: Title: JSON variable file support
  Body: ### Problem
  
  `robot --variablefile burek.json -i smoke ROBOT/` returns the following error:
  `failed: Importing variable file ’burek.json’ failed: Not a valid file or directory to import.`
  
  while `burek.py` containing the same JSON content seem to work just fine. 
  
  `--variablefile (-V)` only seems to support Python and YAML file types. 
  We would like to use JSON file as a variablefile to pass on certain variables that are used by other frameworks as well.
  
  Thanks for all the help!