Multiline keyboard labels don't work if specified in keyboard.yaml

[TwilightKiddy]

While testing #121, figured out that multiline labels don't work if auto_labels: false is present in the config.
By multiline labels I mean labels like "N2": ["2", "@"], where 2 will be lower white line and @ will be upper grey line.
Futher experementation showed that you can't assign such lables via the config at all, it only works if it's an auto label.
So, even if auto labels are enabled, "BackSpace": ["<<", "<<"] won't produce a label with two lines.