Trigger an error if configuration is wrong

[jdeniau]

If there is a configuration error we might want to validate the configuration with json schema and trigger a warning (or an error ?).

example of wrong configuration:

{
  "exclude": ['foo.js'], // typo ("exludes")
  "inexistant-key": "foo", // inexistant key ;)
  "command": 8 // unexpected number
}