JSON Output for Config

[GItThatSparrow]

When providing the --format json flag to the config dump command, the result is not formatted as JSON.

Expected

Expected something like...

{
  "proxy_type":"auto",
  "sketchbook_path":"/Users/nickromans/Documents/Arduino",
  "arduino_data":"/Users/nickromans/Library/arduino15",
  "board_manager":"null"
}

Actual

>>  arduino-cli config dump --format=json
proxy_type: auto
sketchbook_path: /Users/user.name/Documents/Arduino
arduino_data: /Users/user.name/Library/arduino15
board_manager: null

[masci]

Tracking progress in #342, a PR is ready to fix the issue