Preview option support for blackd

[Mogost]

It seems that blackd does not support the preview option now.

I suggest implementing an option for that like it is done for other features https://black.readthedocs.io/en/stable/usage_and_configuration/black_as_a_server.html#protocol

Something like X-PREVIEW would be fine.

I faced this while using intellij-blackconnect and there is an issue about that too https://github.com/lensvol/intellij-blackconnect/issues/37