New console command to manage configuration variables.

```php Thelia thelia:config COMMAND [name] [value] [--secured] [--visible]```

Where `COMMAND` should be `list`, `get`, `set` or `delete`.

For command `get` and `delete`, you should also set the `name` attribute.

For command `set`, you should set the `name` and `value` attributes and optionally add `--secured` and/or `--visible` arguments.
Why did you not test if the config exists before deleting it ? You can have an error like "Call to a member function on a non-object"