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.
It is not needed to create a new instance of Thelia. You can only mock the kernel.

Look the other commands' test to see how it's done.